The Snowstorm Development Viewer version as I write this is Second Life 2.6.4 (225759) Apr 5 2011 10:30:39 (Second Life Development) and can be downloaded from the Latest Development Viewers section of the Snowstorm Wiki. Check the version after download because for some reason I kept getting an older build even after getting the correct one (I kept at it for testing).
While you are waiting for it to download, head to Nyoko Salome's Marketplace page (or go in-world) to get a free "Bounce On Toes" set for animations to test the feature and a basic pre-set to wear.
Yes, wear. A new "layer" is added to your avatar so you can set how your breasts, belly, and bottom react to bouncing and swaying. The TPV feature was one general setting that made every set of breasts behave the same way. Now users can define how firm or saggy "The Girls" are, how much the trunk junk moves around, and how much slosh you have in your beer belly regardless of how others have set their preferences. Of course, those perving you will need to use a compliant viewer, but with The Lab making this a standard, the vast majority of Residents will enjoy (or be offended by) your tastes. Be sure to check all your adjustments with all the anims in your AO and other frequently used motion anims because the test ones from Nyoko only show up and down motion. My poor little pixy alt's butt bounced so much she nearly fell over.
No word on when this goes mainstream, but with the recent emphasis on fast release cycles, I'd say it should be soon. And word has it that the Kirsten's Viewer and the Firestorm Viewer will add this feature, but don't hold me to it. That's all hearsay right now.