Adding this answer to a years old question that is very relevant today. Most new forks today (mid- to high-level ones from the biggest and best suspension makers, namely: Fox, Rock-Shox, XFusion, Marzocchi [owned by Fox], Manitou, Ohlins) are equipped with tapered steer tubes 1.5" at the crown to 1.125" by the time it leaves the headtube and the upper headset. It's getting harder to find straight 1⅛ steer tubes on suspension forks. Frame makers have increased the diameter of today's bicycle headtubes sometimes up to as much as 56mm ID. This is related to enhanced performance and strength of a "stiffer" frame. It also gives designers more room for bigger frame tubes or ones with more aero shapes.
So, while today's bikes and forks are designed for each other, there is left in this world a very large selection of quality bikes that are needing replacement suspension forks which the after-market has essentially ignored. The selection of suspension forks with straight steerers is dearth. And if it's an awesome 26er we're shopping for--good luck with that.
To answer the question: a fork with a tapered steer tube can be used if: 1) internal diameter of the bikes head tube is 44mm or more 2) the lower headset is swapped out for an external cup one whose bearing is sized to accommodate the 40mm of crown race seat of a tapered steerer. The SHIS of such a lower headset may look like: EC44/40. Thus, one can place a tapered steer fork in a bike designed for a straight steer tube by selecting the correct size and type of lower headset.
One other consideration needs to be addressed in these situations and that is the length of the steer tube's taper. Tapered steer tubes are 1.5" at the crown and 1⅛" at the top. The transition to these diameters--the taper-- uses up some distance and the head tube must have adequate length to fit this transition so that the diameter of the steer tube exiting the headtube is 1⅛", which is the size necessary for the upper headset hardware and stem to fit properly. Suspension fork makers utilize a length of 58mm to 63mm for the taper. The bike head tube must be long enough to house the taper.
Example of headset for using a tapered steerer fork in a bike designed for a straight steer tube fork