According to Apple via this
post 1333MHz FSB Harpertown (and likely Wolfdale DP) processors should work in a 2,1 Mac Pro (the clovertown model).
However I doubt anyone has tried it (I certainly haven't seen anyone claim they have) as it doesn't make much sense to go and buy 3.0 or 3.16GHz Harpertowns if you have a working 3GHz 8 core Mac Pro.
The 5000 chipset does technically support Penryn Xeons, it may be a firmware issue on the first Mac Pros; which are the only ones there are reports of penryn being tested on and don't work.
If you already have some harpertowns laying around feel free to try it and report back.