Can I override the HTC Sense "slide down to answer / slide up to decline" screen locker?
Custom roms will be the only way to do this. WidgetLocker changes the lock screen, but i don't think it handles incoming calls. This Gingerbread rom changes the lock screen as well as how it answers calls, changing it to the Android swipe across system, but like all roms, it requires a root and a wipe. It is a great rom though, and will do exactly what you need.