Score 1 for States' Rights!
It's sort of a state right, and sort of a federal issue. Clearly, the Constitution provides that the federal government has more restrictions than rights/authorities, and the people have the bulk of the rights, as they give them up to the individual states:
Amendment 9 - The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.
Amendment 10 - The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
However, when one state says something's okay, the other states have to accept what that state says:
Article 4
Section 1
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.
So, when CA says gays are "married" (i.e., to take as husband and wife, which would be hard to do without two sexes involved), all the other states also are saying that this couple is "married". So, 49 states' rights are denied by one state having its rights.
Now, as stated above, many states require getting a new driver's license, or law license, etc., etc. I don't know of a state that requires a new marriage certificate/license when changing states. Perhaps this is because different states have slightly different driving laws, and very significant legal standards and practices, but for the first few centuries of the nation, "marriage" was a pretty clearly defined term with little or no variance (as stated above, Utah had to change it's polygamy stance to become a state). So, the federal question is whether other states will be required to accept as equally binding for tax returns, wills, divorce procedings, insurance purposes, etc., etc., etc., two husbands or two wives in the same manner as a husband and wife.