I think regardless of your viewing distance the contrast and reference colour reproduction from the Project Kuro should make it a no brainer - you won't be disappointed. Keep in mind most 360 games regardless of whether or not you have the Elite are native 720p, so you won't be gaining much in that arena. Those black bars at the top and bottom of your screen for movies won't be visible with the Pioneer, more of a deep grey with the Panasonic.
It's at your own discretion, but I've stared at both of these TV's considerably, resolution isn't everything...
I would choose the Panny at that distance. With a native resolution of 1920x1080 as a PC monitor you won't be disappointed. Your HD movies will also look great up close at 1080P. Sure the PIO has better blacks, but the Panny is pretty good to, not dark grey either.
I'm using my 42" Plasma as a PC monitor at 1280x768, looks fine but nothing compared to my 50" Plasma 1920x1080, which looks sick btw
The latest Home Theater magazine had an HDTV face off where 50" versions of the sets your comparing faced off.
After all was said and done the Pioneer took first place even though it was the only 768p set in the faceoff.....at viewing distances of 8ft the judges couldn't tell the Pioneer was not 1080p.
Even so they commended the Panasonic as an excellent TV and stated both the Panny and Pioneer are solid investments and or close in performance with the edge going to the Pioneer thanks to it's superb contrast ratio and black levels. They did offer the Panny had slightly more detail than the Pioneer and that it could be a better choice if you sit close to it (like 6 feet away) thanks to it's 1080p resolution.
Me myself I'd get the Pioneer because IMO, nobody has mastered plasma TV's the way Pioneer has. I've seen the latest Kuro line and to my eyes they're hands down the best flat panel sets on the market.