Here in the dark lands of Arch, Steam was broken for quite a few days (as in, it didn't launch) after it ran its own update (ofc might have been something else too), but I also saw some people running Wheezy complain, so I thought this problem must be present in Sid too...
It sometimes can be started
once if you delete your ~/.steam, but not always, so here's the fix that worked for me:
and just wait for the update to finish, then log in (your games are not gonna be deleted).
You might have to kill steam before this, if you already tried to launch it but failed.