What would happen is dependent upon which section of the U.S. Code that Obama cited as his authority to nationalize the Texas National Guard. Title 10 allows him complete authority and Gov. Perry cannot refuse. Title 32 allows him to nationalize only with Gov. Perry's approval.
Title 10 would require BO to put them on full-time, active-duty status paid for by the federal government. So far, he has't been able to get the money he's requested for Border Patrol. I doubt he'd get money to nationalize the Guard.
An attempt to impeach the prick for "misappropriation of funds" would go just about that far.