Update Rollup 6 for Exchange Server 2007 RTM (KB942846) was released on 2/21.
This update rollup is for Exchange Server 2007 RTM only. Exchange 2007 Service Pack 1 (SP1) has its own series of update rollups which are independent of update rollups for RTM. Some issues fixed in this update rollup were not included in SP1; however, they will be included in Update Rollup 1 for Exchange 2007 SP1 based on the current plan.
The download location of this RU is:
http://www.microsoft.com/downloads/details.aspx?FamilyId=0B2993A3-D290-453E-85FE-108867865731&displaylang=en
You may track it via KB: http://support.microsoft.com/?kbid=942846
There is an interesting post over at blogs.technet.com