I've been testing 2008 SP2 for a few weeks now and haven't had any issues with SEP. I plan on upgrading 2 production machines to 2008 SP2 tonight and then upgrading their SEP version from MR4 to MR4 MP2 which seems to work the best.
In testing when I upgraded SEP to MR4 MP2 and then installed 2008 SP2 the SEP client wouldn't start automatically because SP2's installer disables it (can always turn it back on but meh).
If everything goes well and no serious issues arise between now and Friday I'll be upgrading the rest of our 2008 machines to SP2. Well, except for our Exchange server. I'm not very keen on being a SP2 guinnea pig when it's something as important as email. =-)