When switching from BIS to BES on my Blackberry I had some issues that I was not able to find easy answers for on Google, so I thought I'd pass it on to you incase anyone has the same issues (and so I have it written down). My internet didn't work, my personal email didn't work and I ended up with duplicate Calendar Entries.
1. My bookmarks did not work. I had to:

• Edit each bookmarks from "Internet Browser" to "Blackberry Browser".
• Set the default browser to "Blackberry Browser" (Options->Advanced->Browser).

2. After setting up Exchange I was able to get my Exchange email, I did not getting my Yahoo/Gmail email. I also could not get Google Maps or Blackberry Maps to work, I had to:

1. Call T-Mobile and have them flip-a-switch.
2. Add/remove the accounts from the My T-Mobile website. You should get a confirmation message on the Blackberry. (I also re-sent the Service Books from the My T-Mobile website, but I'm not sure if that was required.)
3. Under Tools -> Advanced -> Host Routing Table, highlight the BOLD entry and choose "Register Now".

 3. Since I use to Sync my calendar via USB, my blackberry had all my calendar entries. When BES was setup, I got duplicate entries. To clear these duplicate addresses:

1. Open the calendar and go into "Options".
2. Choose the 'old' calendar and click the Trackball.
3. Change Wireless Sync to "No"
4. Plug phone into computer, go into "backup & Restore".
5. Under Advanced, choose the calendar and click 'Clear'.
6. Change Wireless Sync back to "Yes".

4. When switching from Desktop Redirector to BES, you must un-install the Desktop Software and reinstall and choose the BES option. You cannot modify the existing installation.
5. ViiGo did not work until I downloaded the Enterprise version of it.