I was asked if I defrag my SBS servers.

It depends.

I check with the vendor first of any LOB app I have.

I go around and don't defrag locations where there are SQL databases or Exchange databases.

And Amy of www.thirdtier.net recommends that you defrag Exchange right before you migrate to SBS 2008.  And when she says defrag of Exchange... she means an offline defrag of Exchange

http://support.microsoft.com/kb/328804