It all depends on how quickly you're recycling numbers, I think. $$ will be reset at some point, this is true, but I seriously doubt it'll ever get reset and back to it's initial value within the one second timeframe needed to stop this being a unique ID.

I guess that if you want to be truly paranoid you could look into how the better-coded hit counters work and use that kind of file handling to manage your ID, I think this should work in any realistic situation though.