TechRadar found that the new Nehalem Xeons at 2.8GHz scored well in the SPECfp benchmark with a score of 160 as compared to a score of 90 for Intel's current Penryn-based Xeons running at 3.4GHz.
Intel is expected to finally start selling Nehalem-based Xeon processors on March 29th, 2009, according to Fudzilla.
Intel plans to star selling Nehalem based Xeon processors on what seems to be the last Sunday of Q1 2009. Xeon W 55xx, Xeon X 55xx, Xeon L 55xx and Xeon E 55xx versions will start selling as of March 29th, and you can expect them available as of March 30th, the first working day of that week.
The current Mac Pro was last updated in January of 2008 and is due for a refresh. It is very likely that an update will be announced around the same time as the Nehalem based Xeon processor is released.
MacRumors forum member Tallest Skil has organized an interesting summary post of what he expects in the next Mac Pro.