make a move...call them! u have all the right to ask since they promised u an offer letter. also they wouldn't negotiate for the salary if they were not interested of hiring u. 

but to be sure of the placement, u have to call them! otherwise u'll get stuck waiting for their reply until u lost all other opportunities then at the end u'll realize that all these time u were only waiting for nothing!

they won't bite u if u ask, relax;)