Unlike RDBMS's, MongoDB has a single, shared database-wide write lock when you write to any collection.
Also in practice the limiter is not the global write lock but page faults that cause the largest performance impacts. [1]
1: http://blog.pythonisito.com/2011/12/mongodbs-write-lock.html
That said, as that blog post clearly demonstrates, the write lock is a red herring in a vast majority of circumstances. In practice it is historically rarely been an issue and even less so with the yielding improvements in 2.0 (which is what that blog post is about). Typically issues are manifested in lack of adequate ram, lack of IO and poor schema design.