Doing my monthly inspection & clean of my wheels last night and I was shocked to see a crack around one the spokes as they enter rim. It is a rear wheel (Shimano R600 I think). I had obviously been riding on it for a little while, so I was wondering if there was any urgency in getting the rim replaced, or can I wait until my next visit to my LBS? I currently do only around 100km per week.
if the crack is at the spoke bed, and hasn't spread to the side of the rim yet, it's probably not going to explode immediately (if it's on the sidewall, tire pressure can cause it to burst, which will almost definitely make you crash). However, there's a good chance that the spoke will eventually pull through the rim, and there's also the fact that once aluminum starts to fatigue and crack, the damage tends to spread relatively quickly, which means the rim could fail on you entirely, probably without any additional warning. You didn't say when you next planned to visit the LBS, but I'd recommend taking care of it within a week or two at the most.
A crack in the rim is something which needs to be dealt with fairly immediately. It isn't likely to fail catastrophically, but is it worth the risk of the damage to your body, or more importantly, to the rest of your components?
I'd get to the shop within a few days and avoid riding it very much, at least, until you do. There is really no downside to getting it done sooner, and potentially significant downsides to waiting if you keep riding it.
Can't find a close-up picture of that wheel on the net, to get an idea of its construction. Without seeing the crack, and its relation to the wheel design, it's hard to tell how serious it might be (or even whether it's structural vs cosmetic). A good bike shop mechanic should be able to look at it, though, and tell you if it's a valid concern.
As Matt indicated, the most likely danger is that the spoke will pull through and give you the equivalent of a busted spoke. (Can we say "busted" here, or will the literary police get on our tail?) Anyway, that's an inconvenience but unlikely to result in a crash.
The odds of the crack migrating through to the side of the rim such that a section of the sidewall might separate (or the brake pad grab violently) is pretty small, but real. You should check the crack after every ride, at least (and maybe after every 20 miles or so while riding), to make sure it's not spreading. If the crack spreads over onto the sidewall at all you should stop using the rim ASAP.
(Remember that rims can be easily replaced -- there's no need to buy an entire new wheel if the hub is in good condition.)