Rims are preloaded in compression by the tension in the spokes, and cracks can't really propagate unless the material is in tension some of the time. I'd imagine there's a certain amount of localised tension around the spoke holes, but unless the hole is right by one of those, it shouldn't be much of an issue. It won't cause a catastrophic, face-to-tarmac failure even if it does crack - all that'll happen is a spoke will pull through and the rim will go out of true.

I'd agree that it's something to keep an eye on, though.