OK, since I've had my first real day of work since I got the phone and I rooted it last night, let me report.
Rooted, running Das BAMF 1.6.2 with a custom theme.
I had 98% when I unplugged the phone at 6:50AM this morning. I would consider this a pretty heavy use day, except I do NOT have 4G.
I checked e-mail 6 times an hour, I downloaded about 6 apps from the market, did some IM and texting, showed some friends a video, played with settings, took some pictures (3-4), uploaded them to google docs, made two phone calls (yeah, they actually still do that), DLed Kongregate and played a few games, typed on a forum or two.
Now, as of 10:25 PM, the phone is at 37% charge and has been at that number since before my last phone call over an hour ago.
Battery life on this phone is stunning. I don't expect it to go several days, but if I stuck to just texts and calls, I see no reason why it couldn't go a couple of days on standby.
From reading these and a couple of other Thunderbolt forums, it really seems that there are two types of phones (or batteries) out there. One has good to great battery life and one has poor to suck battery life.
I wonder if it's the batteries or the phones.