Ive got the low-down on entry with CR-1 and IR-1 visas, which are the immigrant visas the US grants to alien spouses (esp. though DCF).
This information is valid only for immigrant spousal visas. The beneficiaries of these visas did not go though the K-1 process and were present and settled with their spouse abroad when the application for the immigrant visa was made.
Spouses who have not had their second aniversary at the time of interview will be granted a CR-1 conditional permanent resident visa. If the spouse then goes on to enter the US on the CR-1 after their second aniversary, they will be automatically upgraded to an IR-1 permanent resident visa. The passport will then be stamped with an I-551 stamp which is the stamp equivilant of a green card. Make sure you look closely at your documents at your POE to make sure they've granted you the right kind of visa.
People upgraded to an IR-1 at their POE, having recieved their I-551 stamp will be free to come and go as they wish as well as work. You should be able to have your passport stamped at your POE with the I-551, and turn right round and fly out of the US again without getting hassle on your return.
People planning to stay out of the UK for more than a few months should consult their Embassy as their time away aproaches one year. If you are out of the US for more than one year once you have been allowed entrance you might not be allowed back in - so better to be safe than sorry.
Apparently people who are admitted to the US on a CR-1 do not have to apply to adjust their status after 2 years. I think a 2 year validity green card is granted and must be renewed shortly before the expiry date. As I understand it, IR-1 beneficiaries are givin a 10 year green card. Both are valid for multiple entires.
This information was gathered from people who have had first hand experience in this issue. I will shortly be able to confim it myself
Hope that helps,
Sarah