Actually.. it does work: here's how.
First step is you need to get the 2008 Mac Pro case. Second, get the 2008 logic board as the 2008 version won't fit in the 06-07 case - ports not lining up or something, also the ram area is different.
Third, get the heat sinks and processors of your choice. You have a choice between the 2.8, 3.0 or 3.2(requires a different board).
Voila! You have a 2008 Mac Pro. Seriously, as everyone on here has told you. It CANNOT be done. If it could be done many of us would have been successful with it. Harpertown 5400s don't work in the woodcrest/clovertown based Mac Pros. Period.