Exchange 2007 is serviced by a new 'Update Rollup' strategy that is designed to simplify testing and deployment. Here's a useful post on the Exchange Team Blog if you need some background.
However, the way Microsoft decided to name the updates will likely cause confusion among IT Pros. Instead of increasing the version number by 1 each time a new update rollup is released, we now have a forked release path. For example, we were at Update Rollup 5 around the time Exchange Service Pack 1 (SP1) released. Shortly *after* SP1 was finalized Microsoft released Update Rollup 6. In the overview section Microsoft clarifies that Update Rollup 6 does not apply to Exchange 2007 SP1. The updates included in Update Rollup 6 that are not included in SP1 are included in Update Rollup 1 for Exchange Server 2007 Service Pack 1 (confused yet?).
I've created a Visio diagram to make sense of the Update Rollups (click here for the .VSD file). Each shape in the Visio file includes a link to the appropriate Update Rollup KB article. I'm also including a PNG version below in case you don't have Visio, or don't want to download the viewer.
Bottom line - once you wrap your brain around this servicing model it makes sense why Microsoft *must* maintain separate release paths. After all Exchange 2007 RTM will be supported until Jan. 2009 per the MS Lifecycle site. My only recommendation is to come up with a simpler naming strategy. Update Rollup 1 for Exchange Server 2007 Service Pack 1 doesn't exactly roll off the tongue now, does it ;)