Phone was taken off charge at about 16:00 yesterday, had both wifi and GPS on. Didn't use the phone for anything, just left it idling with both these on and by 20:00 when I left for work it was at 70%, so 7-8% drain per hour.
Turned off wifi and GPS when I left for work, phone is right now sitting at 60% remaining.
So <1% drain per hour. Could try with just GPS on or just wifi and see what's using it the most, the battery stats (which I checked just before I turned the GPS and wifi off) indicated wifi was the main culprit though.
Bit of a hassle for me as I'm used to having wifi on 24/7 as I have wifi at both work and home (and keep my phone logged in to MSN etc). Suppose I could just use HSDPA for that though.