Hi Charles,
I can confirm 100% what is stated above: there is no incompatability between SEP and Ex2007 SP3. Also, do be sure to use the most recent release of SEP SBE.
Do you also have Symantec Mail Security for MS Exchange on that SBS 2008 server? The product is often purchased together in a bundle with SEP.
If so, here is an article that may be of interest: Symantec Mail Security for Microsoft Exchange (SMSMSE) Compatibility with Exchange Server 2007 Service Pack 3 (http://www.symantec.com/docs/TECH134831) Be sure that you are using SMSMSE 6.5.1, and keep an eye out for 6.5.2 (should be released within one month).
Please let the forum know if your question has been answered or if there is any outstanding matter!
Thanks and best regards,
Mick