So let me get this straight once and for all.

If i get a US version of SC2 and my friends get the EUROPEAN version. we will be able to play together ?

also if me and my friends play ranked games(2v2 3v3..) will we be playing on the US server or the EUR server ?

We are looking into ways of letting you switch between servers, no matter whether you buy a European or American version of the game. As for the second question you won't be able to play in the same game using different servers, as we have never had global servers for our games. It is our goal for the future, but won't be there for release.