In the next few months, Linden Labs will switch the way avatars are loaded inworld. The goal of this change is to speed up the time it takes to render an avatar, and to stop bake fails. The downside of this major update is it will cause a lot of pain until the bugs are all worked out. You will be forced to upgrade your viewer or you will not see textures on your avatar, or alpha masks that hide your shape below a mesh. You will appear as a cloud or grey. Expect bug and/or crashes in all viewers across the board until the bugs are fixed. Many of your favorite viewers, or older viewers, will become unusable because they aren't updated to SSB.
Bloggers will be greatly effected. I personally will loose the ability to take lighting and shadow photographs unless Linden fixes the bug that crashes Macs on the current viewer code, or any viewer that uses the current code.
Check each viewer log for the latest news. Master list of 3rd Party Viewers.
Here are some news briefs:
Firestorm will update to SSB but they warn it will not be quality tested as much as they like before a major rollout. Mac version.
Phoenix 1.5 and Phoenix 1.6 are no longer being updated, therefore will not have SSB. Mac version.
Exodus update on SSB unknown. Has Tone Mapping. Mac version.
Nirans may update only once more, but it is unknown if the update will include SSB. No Mac version.
Cool Viewer is ready to go with SSB. No Mac version.
More news from Firestorm on SSB here: http://www.firestormviewer.org/qa-results