I've got a few customers using Blackberry Enterprise Server (BES) now and they are all running this on Small Business Server 2003.
Research in Motion would probably not recommend this configuration, but actually SBS seems to cope quite well. Although I have not tested this out with more than 6 users.
Installing BES onto SBS is not entirely straight forward, mainly down to the way MSDE is installed on SBS. The BES installation cannot find the MSDE database, but it also doesn't give you the option to automatically install another database instance.
So, in order to get the BES installation working on SBS, you should create a new instance of MSDE before ruining the installation. You can do this as follows:
1. Locate the MSDE files on the SBS 2003 CD 3 or download from Microsoft.
2. Open a command prompt (Start -> Run -> cmd -> clickOK).
3. On the Command Prompt, change to the directory where the MSDE files are located e.g. cd D:\SBS\MONITOR\MSDE
4. Enter the following command to create the new instance:
setup INSTANCENAME="BESMgmt" SAPWD="AStrongSAPwd" /L*v C:\MSDELog.log
5. Start the service: Start -> Run -> Services.msc -> Click OK. Scroll down to the instance you just created MSSQL$BESMGMT, select and click start service.
6. Check the log file to make sure everything installed correctly at C:\MSDELog.log
Then when you run the BES installation, simply enter BESMgmt as the database name. The installation will then run like a dream.
Make sure you follow the Quick Start Guide for the rest of the installation available here: BES Express 4.1 Getting Started Guide