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It appears you have already done quite a lot of research on his residency. I agree that your husband would meet the presence test in May and be taxed in the US on worldwide income starting in May 2011.
The US will allow a foreign tax credit on form 1116 to help offset the UK taxes associated with the double taxed income.
I recommend discussing with the UK tax accountant or review UK tax return to determine how much of the income is taxed and apply this to the US return.
To answer your final question, I agree your husband will be a dual status taxpayer in 2011 for US taxes and will need to file a 1040, with a 1040-NR attachment to report any US source income on the 1040NR and will need to pay tax on this income. Be sure to write Dual-Status on the top of your return for 2011.
Please let me know if you have any additional questions, or if I did not answer yours completely. It appears you are looking more for confirmation and I would agree that you have it all right, if this is what you need.
Thanks. I wanted to make sure I was on the right track.