WHen Mitt Romney is elected President of the United States of America, he will not attend Church services at any LDS Temple. Church services are not held in the temple, nor are they closed to the public. Mitt would attend services at his Wards (similar to a Catholic Parrish) meetinghouse. 
 
Temples are closed on Sundays, there is no ordinances performed in the Washington D.C. Temple Sundays or Mondays. Now Tuesday through Saturday the Temple is Open and when the President attends the Temple his Secret Service attachment cannot enter the temple unless they hold a Temple Recommend like Mitt Romney does.  
 
Even if they held a Temple Recommend they would not be permitted into the temple while armed. Weapons are not allowed inside the temple it is a sacred place a place of peace and worship of God.

^ What he said :D

Generally speaking, in the LDS church, you're not really supposed to pick your congregation. Instead, every geographic area is divided up into "wards" and so you'd just attend whatever ward in which you reside. So technically the White House is inside the geographic area of some ward in the DC area, and so somewhere nearby there would be a chapel where people in that ward attend. If Romney were to win, that's where he'd attend. When traveling, you just locate a nearby chapel and attend there. 
 
Sunday meetings are open to the public, so the Secret Service people along with any of the President's constituents would be welcome.

Dave you are absolutely correct about the assigned ward but an exception could, and in this case I suspect would, be made, this requires First Presidency approval. In addition this restriction only applies to where your membership records are kept, you can actually attend where ever you wish, but you can't hold a calling etc. unless you records are there.