Is it safe to ride on a cracked rim? Doing my monthly inspection &amp; 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.