I think it does check upon coming online, as long as it's been 24h, or whatever you set, since the last check; Texrat indicated otherwise, and I could be wrong; mine is almost always connected, so I don't really know.

If it doesn't, then it definitely should. (Provided the timeout has elapsed -- if you're auto-reconnecting in an area of flaky data coverage, you could wind up updating every 5 minutes, which does eat bandwidth to no purpose.) It'd be nice, also, to provide UI for the timeout setting; right now it's done with a manual gconf edit, analogous to a registry tweak in Windows.