The best thing is do your own test, on a loop where you know how fast you can really go on a 26 - judge for yourself.
Other have done the same - recently is Sauser. (scroll down to his entry from 21.01.2011) - 29er won, Up and Down.http://sauserwind.com/diary.asp
SingleTrack.com has done some tests - but not very well - read to find out why (no standing out of corners???)http://singletrack.competitor.com/2011/ ... -dead_3050
I have the luxury of both right now, they are REALLY close on most tracks (within seconds). Both bikes are within 1/2lb, same tires most of the time for training (Ravens 2.2s).
REALLY tight trails, I will give a small advantage too the 26, but everywhere else, 29er. Up, down, smooth, rough, traction, stability - it's just a tiny bit better at everything. I have done my own back to back test a few times on a few different trails.
The one time the 29er had a HUGE advantage was on a long (for here) very rocky and rough, but moderate, middle ring climb. The 29er was about 20-25 seconds faster on a 5 minute climb... I couldn't believe it, so I repeated on both bikes - same results. After that, I was convinced.