The IRS is going by the date of marriage, so if you amend a separate return (eg, a single 2008 one), you can only claim $4000.
So, either submit TWO 1040x forms or wait until 2009. I'd wait until 2009.
At this point, wait until you do your 2009 return and put it on that return.
The turnaround for the amendment return is now averaging 20 weeks. And this will only slow when the IRS turns its gears to processing normal returns come January. The 20 weeks puts you in March. Put it on your 2009 return and you'll get it in February if that's when you file.
Going on the assumption they are both eligible for the credit, then one of the married individuals can claim the entire credit on an amended 2008 return. If one is not eligible, then both are not eligible because they were married at the time of purchase.
You can do it either way. It's probably simpler to do just one amendment - and if one spouse can't trust the other with the $8000 the marriage is in trouble.
you didn't get married or buy a house in 2008 - it has nothing to do with 2008's tax returns - there is only ONE $8000 credit per house. - you will get it when you file your 2009 tax return
You can't amend a tax return because you bought a house. You take care of it in next year's return.