I welcome Adam's change of heart on the federalism issues with DOMA, which he expressed in his post earlier today.
As a staffer on the Senate Judiciary Committee at the time of the law's passage, I didn't think this provision a hard question. The federal government can define the terms of its own laws, just as the states may for their own. Congress can define marriage for federal tax and benefit law, just as states can for their own laws. Federalism forbids the feds from forcing states to adopt a definition of marriage as one man and one woman, just as it also forbids states from forcing federal law to define marriage as between heterosexuals or homosexuals.
To see this point clearly, suppose a state defined marriage to include polygamy. That would bind the state, but the federal government would be under no constitutional obligation to accept the definition, and it could keep marriage as a man and a woman.
For some more thoughts on the gay marriage cases before the court, here's my appearance from earlier today on the Wall Street Journal's Opinion Journal: