It can be done, but whether you'd want to or not would depend on what the SBS server was used for and which BES components you want to install. Search this forum for "SBS" and you'll get the flavour of the arguments for and against.
One company I work with has it running successfully for the past 9 months. They're supporting about a dozen clients, 5 or 6 of which are BlackBerry users
I run BES 4.1.2 and so does a couple of my customers on SMS 2003 with less then 10 users.
I believe it is important to note that the SBS servers are very high powered computers that are less then 5% busy at business day peaks. This is why I think it works rather well.
If the server was going to be very busy I would put BES on its own server.
Install BES on SBS 2003
