When - Next Sunday
RLVa Requirement will be turned on next Sunday. If you log in with a viewer without RLVa you will not be able to participate on the sim.
Why - Privacy
For a long time we have been unable to obtain privacy within our sim, with users being able to peer-in, sit-teleport into private conversations or see the whiteboard in private meetings among other things. It has always been a problem.
The issue became further apparent recently as we begun to develop student accommodation within the sim. Students should be able to shut the door and switch off rather than feel like there are still people making what they are up to their business. It is impossible to employ standard SecondLife parcels for privacy features due to our sim layout. We made several feature requests    to Linden Lab to try to get better parcel privacy features without success.
Ultimately we tried to avoid requiring RLVa for a long time, as we know it prevents people on the default viewer or people who simply don't want to use RLVa from participating on the sim. But it is felt that it is holding us back too much, preventing enjoyment of the sim. By requiring RLVa we are able to set a camera distance limit and prevent sit teleports, allowing guests to enjoy privacy within the sim.
Much like emails, so long as you understand RLVa it is safe. We've compiled an article which explains RLVa with links to further reading if desired. We recommend reading it if you are not familiar with RLVa, before deciding to enable it.
Enabling RLVa in Firestorm