Curious fact: I used to be able to do this (clear defaults for HTC Sense, press home, select Launcher2) in earlier alphas of the Desire ROM. However... ever since Paul integrated the Desire release candidate, you can still do this but there's no more Launcher2. Nada. Zip. Gone. No Launcher2.apk in the ROM period.
Anyone who's got a real Desire wants to confirm/deny?