By all accounts, Apple's "Mountain Lion" operating system will be released today (Web), a bargain at only $19.99 for a slew of features (Web). However, RoaringApps reports (Web) that the Second Life viewer is NOT compatible.
Note that they list a slightly out-of-date version of the SL viewer, the current being 3.3.3 released to the wild on June 21st. Whether this assessment is valid or not depends on your exact configuration, of course, as a peek at the SLviewer System Requirements (Web) shows. Based on that information, my 27" iMac shouldn't really be able to access SL even now and yet I do. The only third party viewer I found in the list was Phoenix and that was listed as having questionable compatibility.