I petitioned for my husband to become a US resident and we just finished the process and will be getting the green card in a few weeks. Up until now we have filed separately on our tax returns (including 2007), but I am trying to figure out things for 2008. My husband will have to start to pay taxes for this year and so once his green card arrives can fill out tax forms through work that will tax him (as opposed to being exempt). We will probably file jointly for 2008, but what do we do since he was a nonresident for January through March and now will be a resident? Any helpful insight would be appreciated.

Once her gets the green card, he is a resident and files as a resident, PERIOD. The previous time as a non-resident alien is NOT CONSIDERED.