In the words of sponge himself:

19:59.47 <sponge-> if anyone wants to try out the new quake live, just right click
        quake live on steam, properties, beta, and opt into the beta branch
19:59.54 <sponge-> we're basically at a near release candidate state!


Goodbye friends, standalone master race salutes you!

Edit: check also our forum topic QL Steam switch Q&A with SyncError.