This all varies depending on the rules of the scheme you join, but for final salary pension scheme I am in through my work, because I am contributing on a pre-tax basis, if I do not stay for at least two years, I lose all of my money that I contribute and cannot transfer the pension elsewhere. Once I reach two years, I can leave the pension in place through the same scheme until retirement (though if I only stay two years, the payout would be a VERY small annual amount and it has been recommended that I keep a UK bank account open if possible to make things less complicated). I was sent a list of Qualifying Recognised Overseas Pension Schemes that I can also transfer the money to once past the two year mark--there are about 12 or 13 options for the US,the most recognizable of which seem to be Fidelity's 401K plan and a Citibank IRA.
What I have not yet researched, however, are the tax implications in all of this. I will need to find out more before deciding whether it is best to keep the money in the UK or transfer to the US.