I did a quick survey thanks to the world wide web of good 29er bikes. Both the fully suspended variety and the hardtail type.
Like 26" MTBs in the late 80s (when they were new to the market) the dimensions are all over the place.

Hardtails:

Gary Fisher Superfly (and the rest, including my Rig) 440mm
Avanti Scratch (the review bike that got me into 29ers) 450mm
Titus Fireline (the nicest hardtail I've ridden) 453mm
Niner AIR9 and ONE9 440mm

Duallies:

Gary Fisher Superfly 100 452mm
Santa Cruz Tallboy (hopefully my next new bike) 445mm
Pivot Mach 429 456mm
Niner WFO (the freeride choice of the 29er set) 455mm
Specialized Stumpjumper 29 448mm
Ventana El Rey 465mm

So there really isn't that much in it. Between the shortest of the hardtails and the longest of the duallies is only 1 inch - 25mm.
Interesting to me is the fact that the Tallboy with 100mm of wheel travel is only 5mm longer in rear centre than the Rig, and 8mm shorter than the Fireline.

I guess this doesn't really teach us that much. Which isn't surprising really - wheelbase is only one aspect of frame design that affects handling, and rear centre is only half the story with wheelbase.