Is it safe to ride on a cracked rim?
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.