There have been rumors for YEARS that Cheney was going to step down. Since he agreed to run in 2001, basically. Remember all during the convention in 2004, they kept saying that he was going to step aside? Cheney's not going anywhere. The day after the SOTU, the President always goes somewhere to reiterate his message. The fact that he picked Tennessee means nothing.
It was pointed out on that other thread that the Gov of TN is a democrat. If Frist left his Senate seat, he would be replaced by a Dem.
That, added to everything else, makes it make NO sense that this rumor is true.