Howdy - I will be upgrading our exchange from 2003 sp2 to 2007 sp1 running on a 2008 64-bit platform. If I'm reading this thread correctly, it is probably best to perform the following steps - please let me know if you think otherwise:
1. upgrade bes from 4.1.3.16 to SP6 (4.1.6) - this is my first BES responsibility - these SPs don't cost anything, right? Free to download and install?
2. install 2007 SP1 on 2008 server
3. remove ESM on BES and install Microsoft Exchange Server MAPI Client and Collaboration Data Objects 1.2.1 (v 6.5.8039.0)
4. start moving folks over to the new exchange server

I don't know if removign ESM on the BES will break it for the folks that aren't moved. What about that?

Thanks a lot for your help!