Mystery money from US Treasury

Today I received a cheque for $400 from the US Treasury. I didn't request the Making Work Pay Credit and I wasn't due a refund, so I'm not entirely sure why I've received it. Should I send it back, or is it ok to keep it?

I had a professional do my taxes and he didnt apply for it. They sent me a letter saying I was due this and refunded it to me.  I asked the tax guy and he said well they reviewed the tax return so you are due it.
I am hesitant and holding it in savings as I am waiting for them to ask for it back ;-)

Yes, that sounds like the Making Work Pay.  My colleague at work (here in the US) had never heard of it, never applied for it, they sent him a cheque for $800.  I laboriously filled out the form and applied for it in the correct manner.. what was the point, if they automatically gave it to people who did not apply for it??

Update on this, today I got a letter from the IRS stating that they had looked at my return and decided I was entitled to the Making Work Pay credit and would be receiving my cheque in 4-6 weeks. I guess this letter was supposed to come before the cheque, but at least I know have proof that I can keep the money!

Oh well - all you have to do now is remember to report it on your UK tax return for 2011/12.

What? I have to report a US tax credit on a US tax return? Where?

This is foreign income and goes on the foreign pages of the UK tax return. 

Unless you use the remittance basis of course, in which case no UK tax is due for most folks.

Oh right, ok.