And your source is? Only 2006-2007 Mac Pro has 32-bit EFI so IF 10.7 has only 64-bit kernel, then it should work on 2008 Mac Pro as well.
It is not about "it should work" on the hardware. 10.6 could have worked on the about 3 year old (when 10.6 went into developer release) G5's too. It is about whether Apple puts the machine into the QA regression test set for the release and subsequent updates. If 10.7 "betas" are not tested on 2008 models then it will not be released for it. ( it might work with some hacking, but it won't be supported).
It may boil down to numbers. There may be not enough 2008 models to make it worth the extra expense. Additionally, the release date for 10.7 may move the 2008 models on to the "too old" pile.
Apple kills off support for older machines. 10.7 will probably release around Q4 2011 or Q1 2012. At the latter point, a 2008 machine is minimally 4 years old. Is that machine even being supported by Apple anymore? Even with Applecare you are out of the window. If on a 3 year depreciation schedule, then at 4 years you are past zero. On a 5 year schedule, you only have a year left.. ( should be far more worried about migrating to new box than installing a brand-spanking-new OS. Need a stable current system more than a new one. ). 2007 is definitely out of the range Apple will support just on age not hardware capabilities. The more the 10.7 release slides into 2012, the 2008 machines will be just as "too old". (the MP release date in 2008 was Jan 2008. That means 10.7 need only slide one month into 2008 before all the 2008 models are "too old". Somewhat likely the "stable enough for production" version 10.7.2 won't even be out by then even if did release in then. Definitely would be "too old" for all the subsequent dot releases. You can't just look at the release date. Apple isn't going to stop supporting a model with the 'dot' updates. Once on board at the beginning supported till releases 10.8. that is another two years. ).
You can see where Apple is moving to a model where users get one, maybe two Mac OS X upgrades before they drop support. That's because Mac OS X is on a 4-5 cycle for those two updates if measure "first release" (on first) to "stable release update" (on second). Apple openly said a couple years ago they were going to slow down the pace of Mac OS X updates to a somewhat slower pace. 2+ years is likely what they are going to settle into.
Mac Pro 2009 and up would mean that pretty much all non iX Macs wouldn't support it and that sounds ridiculous because Apple still sells computers with C2Ds.
No, they could just cut Mac Pro 2008s and leave the a large body of C2D in. All they have to do is cut the opaque misdirection pointing at CPUs when this is really about cutting off old hardware system platforms. Apple is trying to use CPUs to date the platform (hence the xxxMHz sometimes. ) which is imprecise.
The 2008's would get security updates via 10.6. They will be in "extended life support". However, at end of 10.7's period as lead release, it would be around 2013-14 and the machines are 5-6 years old. That's a long time in "computer years". Users are free to keep using machines that old, but the primary purpose should be to look in the rear-view mirror running "2011 and previous" era software. 10.6 will run the vast majority of those just fine.