Are U.S. start-ups able/willing to help foreign (European) developers get an employment visa, or is this too much hassle for a smaller company? Have you done it in the past?
EDIT: I rephrased the question, as it was meant to be general. I myself am not going anywhere for at least 1 year. I am merely planning my career on a long-term basis during this weekend.
From the employee side there's also risk, while you can switch jobs on an H1B it's not trivial and startups have a tendency to go under (luckily my current employer is kicking ass so my chances of an unemployment related deportation are very close to 0 but it still worries me at times). Having your company go under is never fun, that coupled with facing a deadline to find another job with visa related friction slowing the process down is not something I'd like to try.
On the upside working for a US startup is an amazing experience. Ireland has some hardy souls trying startups but it's like another planet over here. A lot more energy and enthusiasm around starting something new and people don't treat startups like a last ditch attempt at employment which is nice.