It's not unusual for a firmware/system update on any kind of mobile device to require a certain level of battery charge to install, but I didn't know that an update on the HTC EVO 4G won't install even when it's plugged in and charging if the battery is too low.
Some devices won't update at all unless running on AC power, which I've always found annoying (especially when the battery is at 100% and is at no risk of running out while the new file is being installed), but I would think that gadgets that can be updated when running on battery power wouldn't need to be above a certain level when plugged in.
As you can see from the screen capture, my EVO's battery was at 17% when I was trying to apply an update (version 1.32.651.6, the one that was released on launch day last month; I'm starting from scratch with my second EVO, remember?). It was also being charged, as evidenced by the charging battery icon in the status bar.
I was able to install the update when the battery was at 27%; I wasn't able to at 21% so I assume 25% is the magic number.
So if you want to apply a new update to your EVO and think you can do it while the phone is charging and the battery is less than a quarter full . . . you can't.