Add me to the list.. (didn't have time to read the posts, time to go home, sorry). And didn't find this before (I've been on here for 5 years :-O )
I've mostly noticed it on one right-turn onramp that's uphill on the way home from work. If I gun it there before straightening out, it clunks. And more recently (been colder in the Bay Area), when car is cold, really gunning it (getting cut off in traffic) has done it a few times when going straight.
It definitely sounds like it's coming from the driver's side at the back. I've always written it off as the exhaust banging around (so I wouldn't worry myself to death).
'05 LGT 5EAT, stage 2/3 (bnr16g, injectors, fuel, gutted pipes), Cobb sways on soft, Koni shocks around medium, stock springs.