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Shimano sells Hollowtech II bottom brackets which are labeled for road and mountain bikes explicitly. I had no trouble fitting a Shimano Deore BB52 Hollowtech II Bottom Bracket into my road bike and so I wonder is there a real subtle difference that I failed to notice, or is this just marketing?
I'm going to limit this answer to Shimano Hollowtech II BBs as per the question. Some other brands do it a little differently.
Mountain external bearing crank spindles are longer than road spindles, and their respective bottom brackets are sized to fit one or the other. Shimano BBs have a plastic sleeve joining the two cups that protects the bearings from water dripping down from the seatpost opening and other contamination. That sleeve is a fixed length, so that if it's to be used the cups have to be that far apart from each other (give or take a tiny margin for variance from the exact nominal shell width, i.e. 67.95mm). This isn't the only potential problem with mismatching mountain and road, but it is the first one that will stop a bunch of the potential hack-type workarounds involving adding or taking away spacers from being able to work.
In the beginning, when external cranks came out, road bikes mostly (BB30 existed in nascent form) all had 68mm threaded ISO shells (70mm for Italian) and mountain had either 68 or 73. E-type (bracket-type in the nomenclature of the image below) front derailers and chain retention devices also have to be accomodated by MTB bottom brackets, which is done by keeping the drive side cup thinner than it could otherwise be and running a spacer if they're not being used. (E-type front derailers and many chain retention devices mount clamped in place under the right BB cup.)
Road external cranks and their bottom brackets don't use any spacers between the cups and the shell. Mountain external cranks have longer spindles and the bottom brackets generally come with spacers to accommodate both 68 and 73mm shells, each with or without an E-type front derailer or chain device under the cup. You're using some amount of spacers in all configurations other than 73mm + e-type FD or chain device, and the chainline stays the the same, regardless of shell and spacer configuration. (Complicating matters is that I believe there are some Shimano mountain BBs that don't have seperate spacers for e-type or chain devices, but still do use 2.5mm spacers to adapt for either 68 or 73mm shells.) And road chainline is a different, smaller number, again the same regardless of ISO or Italian.
(From Shimano service instructions for many external MTB BB models)
What can be made to work presuming a willingness to forego the plastic shield (which actually does a lot of work) and/or add spacers not supplied is another question.
To be clear, a 68mm shell is the same on both a mountain and a road bike, and, other compatibility issues such as chainring clearance and chainline notwithstanding, either type of crank with its associated BB can be installed on a 68, say for the purpose of putting a road crank on a hybrid to make it more "road" or a mountain crank on a road bike to make it more "touring". 73mm shells require mountain cranks.
To put it all another way: Say you've got a road crank, a mountain BB, and a 68mm shell. If you install the BB into the shell as intended, you'll find that the tip of the spindle poking through the left cup is inadequate to clamp the left crank onto, because it's physically shorter than a mountain crank. If the shell was 73mm, the situation would be exactly the same, although you would have used fewer spacers to get there. If you tried not using the spacers, the physical length of the plastic shield would keep the cups from being able to torque properly against the shell. If the situation were reversed, say a mountain crank, road BB, and 68mm shell, if you install the BB as intended than you have excess spindle sticking out of the BB. If you add spacers under the cups to take up the slack, the plastic shield becomes too short to fit inside and doesn't do its job properly.
The thickness of the external parts of the cups themselves, measured from the shell contact to the crank contact, is also not the same between mountain and road. Road ISO is about 11.2mm and mountain is 10.2mm. I believe the reason for the difference is that road handles its two different shell width (68 for ISO and 70 for Italian) by having the Italian cups be 1mm thinner in this dimension. Mountain handles the different shell widths (68, 73, plus the need to accomodate e-type and chain devices) by allowing different spacer configurations.
Some other companies, namely SRAM/Truvativ, work differently because the plastic shield slides within the cup, creating a situation where the same Truvativ/SRAM Team GXP for ISO is the generic replacement BB for almost all SRAM and Truvativ external bearing cranks for ISO threaded shells, mountain or road. It comes with two 2.5 mm spacers to be used in the case of a mountain crank on a 68mm shell.
Another detail with a small difference is the lengths of the threads on each variant. Shimano mountain bottom brackets have longer threads, at least on the drive side, to account for the spacers you would use for a mountain crank, particularly in a 68 mm shell.
I found a blog post with some interesting measurements. I can no longer see it but there was a good photo of both bottom bracket (BB) types together, clearly showing the longer threads on the Mountain drive side cup.
Fortunately, you are using a mountain BB in a 68 mm shell and have plenty of threads engaging (more than you would with a road BB). The minor negative might be the 10.2 mm external cup width mentioned by @NathanKnutson being smaller than expected by the crank length. If you have sufficient preload then I guess you've got away with it.
The alternative situation would be more of a problem. If you used mountain cranks and spacers with a road BB, with its shorter threading on the drive side, you would have fewer threads engaging with the BB shell.
