Back in May, Linden Lab said it was considering dropping support for Mac OS X 10.3 (Panther). Well, the deliberations are over, and the Lab has just announced that that is what is going to happen.
Development for Panther 10.3.9 will cease, and no more QA will be done. If you are one of the few remaining users logging into Second Life with this venerable operating system version, you'll need to stay with the 1.19 viewer series. Eventually, that's going to stop working -- and you won't have access to many new features (like the upcoming Mono).
Support for all other supported systems (Windows XP, Linux, Windows Vista, and later Mac OS X versions) is not planned to change.