As soon as I was invited to the BETA I showed my support and got SS3 (well it helped it was on sale right ;) ). As can be seen from my profile I have been playing it quite a bit since then but now all of a sudden after one day in which I made no changes to my system I am getting:
SteamAPI_Init() failed; SteamAPI_IsSteamRunning() failed.
SteamAPI_Init() failed; unable to locate a running instance of Steam, or a local steamclient.dll.
Neither SS3 or the world of Goo work now and both spit out this same error. I disabled the in game overlay, copied the steam api library to lib32 (changed the permissions), attempted to launch the games from both the steam gui and from the command line but its the same thing every time.
An ldd on the Sam3 binary shows that all the libraries are found and the same for libsteam_api.so.
The wierd part is that I recently did a major update to my system which included upgrading my kernel, a ton of libraries and also my NVIDIA drivers but SS3 still worked afterwards and in fact received a 15 fps boost.
I can confirm this same error on both kernel 3.4.9 and 3.5.7 (gentoo x64 sources) with either NVIDIA 310 or 313 drivers and a 560 2G card.