Two weeks ago, a new software upgrade (version 3.30.651.2) for the HTC EVO was discovered by devs and quickly baked into updated custom ROMs. There weren't any official release notes accompanying the leaked RUU, of course, but a new kernel and hboot, updates to the camera, and various system changes were all found to be present.
If you didn't flash the RUU or an updated ROM (I've got SteelROM RevH on mine), then you've undoubtedly been waiting for the new software to be released OTA. Maybe you've even been mashing the "Check now" button in the system updates setting.
If so, then you should stop now. Version 3.30.651.2 won't be rolling out to your EVO.
According to Sprint, the new software is "required only to support a manufacturer part change on new EVOs." In other words, only newly manufactured EVOs need the new software. The carrier insists that there's no difference in user experience between the two versions and that both are equally eligible for all future software updates.
I've been running a 3.30.651.2-based ROM on my "old" EVO for the last two weeks without issue—and so have a lot of other people—so I don't see why the update isn't being made available to everyone. Yes, old EVOs do not require the update, but they can run it just fine. I would think that releasing it would prevent any confusion caused by having two different "current" versions co-existing, but I guess Sprint reps have already gone through training to deal with customers calling in to ask why their friend's new EVO has a different software version on it and how they can get it on theirs.[EVO Maintenance Releases @ Sprint] Thanks, Dev!