I think the Quadro FX 5800 is very likely since the Mac Pro is missing a high-end GPU option. The 512MB HD4870 is a mid-range card with a MSRP of $150 so it should have replaced the HD2600XT not the 8800GT. Personally, I'm hoping that Apple adds the upcoming 1GB HD4890 alongside the Quadro FX 5800 since the HD4890 is a true high-end GPU.
The delay with the Quadro FX 5800 probably isn't engineering resources on Apple's part but more availability of the 55nm GT200b chips since I'm guessing nVidia is concentrating them on the desktop market to beat back ATI's HD48xx series.
In regards the 3.2GHz model, if it's introduced it'll have to be as an option on top rather than directly replacing a previous model since Intel has no near-term Xeon price drops planned. People shouldn't have much reason to complain since there would be no changes to existing configurations.
Heat shouldn't be a big issue. The 3.2GHz Harpertowns used in the previous Mac Pros had 150W TDPs for 300W total. Dual 3.2GHz Nehalems with 2x130W TDPs would still have breathing room.
Personally, I'm still disappointed with Apple's overall CPU choices for the Mac Pro. The entry level Nehalem Mac Pro uses a 130W TDP 2.66GHz W5320 which costs $284 (
http://ark.intel.com/cpu.aspx?groupID=39718&code=W3520). In contrast, the previous entry level Mac Pro used dual 2.8GHz Harpertown E5462 which cost $797 each (
http://ark.intel.com/cpu.aspx?groupID=33084&code=e5462). Since Intel has each price point representing a performance class, Apple is choosing significantly lower-end CPUs for the latest Mac Pro, but I can't see them making changes now.