Not long ago, I met with the founder of a b2b software startup in London. He wanted to pick my brain: now that they're somewhat established in Europe, he wanted to know how they could 'crack the US' and what sort of partners he should try to meet on an upcoming trip. He went on to state that entering the US was the most important thing for his Company (which I would agree with).
I challenged him as to why they weren't moving themselves to the US. If they're that serious about the US being their most important market, they would probably be best served to physically be there and not go through partners.
And finally, here's the thing: "cracking the States" is hard for European companies. There aren't that many EU software or media companies that have successfully rolled out in the States. MySQL & Skype are exceptions that come to mind but otherwise I'm drawing a blank…maybe Last.fm. Spotify has potential but still hasn't launched. This doesn't mean that European companies shouldn't try but they should go in with their eyes open and be ready to go 'all in' if they're serious about it.