For people that are running into the bug on startup that looks like:

"Map Version is 0, expecting 19"

Try this to see if it works:

1. Go into your \Steam\SteamApps folder.
2. Rename the file called sourcemodels.gcf to something else.
3. Relaunch the game.

This is going to require a redownload of this file, but if you want to get playing tonight it's probably the best path. There are a couple of people now that were in this situation that were fixed this way.

As soon as we possible we'll release a fix for this that will not require this workaround.

Please let me know if this does or does not fix this specific problem for you. Just shoot me an e-mail at erik@valvesoftware.com. If this does fix it, it would be useful to get copies of a couple of the renamed (bad) cache files.