Support for Mac OSX 10.7.x (Lion) came with the release of 12.1 RU1 as previously stated. The issue is that there were "fixes" needed on the SEPM side of things to have them be "managed" successfully.
Until you have updated your SEPM to 12.1 RU1, protect your Lion based Macs with a self-managed client. When you have updated your SEPM, convert them to be managed by following:
http://www.symantec.com/docs/TECH131585
MJD
P.S. The upgrade of a single SEPM to RU1 takes about 10 minutes and does not require a reboot.