Well, I've been fiddling with the SDK, Hammer and whatnot, and I managed to convert a mod created with the 'create a mod' wizard in the Source SDK Launcher to a 2013 mod. I got it to appear in Hammer next to the HL2, Ep1 and Ep2 entries. I was also able to successfully compile a map with the latest compile toolset that comes with the Source SDK Base 2013 Singleplayer.
Look at how new vvis is for example:
** Executing...
** Command: "D:\Steam\SteamApps\common\Source SDK Base 2013 Singleplayer\bin\vrad.exe"
** Parameters: -game "D:\Steam\SteamApps\sourcemods\spaceshuttletribute" "D:\Steam\SteamApps\sourcemods\spaceshuttletribute\mapsrc\shuttle_test"
Valve Software - vrad.exe SSE ([b]Aug 29 2013[/b])
That's just 8 days old, and so very up-to-date.
And yes, I do realize that Source 2013 is merely just an update that adds support for Max and Linux, but I like to keep things up-to-date and who knows what has been added/fixed in this release.
The only problem I bumped into is that textures, models and entities in the Hammer 3D view appear very blurry and low res:
Ingame it looks fine though.