The line "Britney Spears Federline, a gym!" was put into the script at the last minute, two weeks before Spear's marriage actually happened. They were guessing she wouldn't have another 48 hour marriage and the joke would fly through.
In this episode Karen states that in nine years of working for Grace Adler Designs, she has never cashed a paycheck. However in the episode 'Polk Defeats Truman' (season 2, episode 5), she cashes all her paychecks because her husband puts her on a budget, leaving Grace broke.