In the interest of giving implementers enough time to update their implementations, I'm announcing the _rough_ (and always subject to change) schedule which we at Google will follow for the SPDY/2 to SPDY/3 update.
Chrome 19 will support both SPDY/2 and SPDY/3. Note that Chrome 19 is currently in dev channel. It is as yet undecided whether and how often SPDY/3 will be preferred over SPDY/2 when a server advertises support for both via NPN, because we are trying to make sure not to regress performance with SPDY flow control. Obviously at some point when we are happy with it, SPDY/3 will always be preferred over SPDY/2. Hopefully this can happen in the Chrome 19 release cycle, but might happen in a later release. Once it is stable, we will most likely retain support SPDY/2 for a release or two (this is still TBD), but we want to encourage servers to switch over to SPDY/3 support, so at some point we will drop support for SPDY/2.
Google servers will advertise support for both SPDY/2 and SPDY/3 sometime in March. Clients can begin testing against Google's SPDY/3 implementation at that point. It's likely that we will support SPDY/2 longer on Google servers than on Chrome (not all clients will auto-update, such as Android phones using a Android browser version that speaks SPDY/2), but we again will kill off SPDY/2 support at some point, so please transition to SPDY/3 implementations soon. We've spoken to Patrick McManus on the Firefox front and he will likely begin work on SPDY/3 as soon as there's Google server support so he can test his implementation. So we hope that Firefox will switch over to SPDY/3 in not too long, which should, combined with Chrome, account for the vast majority of SPDY use on the client side.
Basically, the important thing to take away is that Google Chrome and servers will begin deploying SPDY/3 implementations in March and hoping to ditch SPDY/2 support as soon as we feel it is "reasonable" (which is very much TBD). And we want to encourage other implementors to switch over soon, and thus are declaring our intent to drop SPDY/2 support at some point in the future, so don't dawdle too long :)