Second Life has worked just fine on Lion for months (and they fixed a lot of the driver issues, too). I've been using it. This is true of versions 2.3, 2.4, 2.5, 2.6 and 2.7 (and 2.7.2), as well as Firestorm and my own work. I haven't tried anything else.
Second Life 2.7.2 (233432) Jun 21 2011 17:17:16 (Second Life Release)You are at 297647.0, 255774.0, 24.1 in Andretti located at sim21001.agni.lindenlab.com (216.82.30.35:13006)Second Life RC BlueSteel 11.06.28.234196CPU: Intel® Core i7 CPU 870 @ 2.93GHz (2930 MHz)Memory: 12288 MBOS Version: Mac OS X 10.7.0 Darwin 11.0.0 Darwin Kernel Version 11.0.0: Fri May 6 22:16:19 PDT 2011; root:xnu-1699.22.53~1/RELEASE_X86_64 x86_64Graphics Card Vendor: ATI Technologies Inc.Graphics Card: ATI Radeon HD 5750 OpenGL EngineOpenGL Version: 2.1 ATI-7.0.82