With John Thune not running, Chuck Todd and others quickly noticed that no Republican Senators are currently running for president, which is highly unusual -- according to Todd, it would be the first time since 1904 that no Senator ran (I haven't double-checked that myself; note that candidate lists before 1972 are difficult to generate, because candidates didn't necessarily have to take any explicit, clear actions).
I still think there's an excellent chance that Jim DeMint will enter primaries next year, and at any rate I'd say that both DeMint and Thune have run for president this cycle -- they just aren't going to be running in by 2012, or at least Thune will not. There are also a couple of other Senators that still could be late entries, too.
But putting that aside, why hasn't the Senate produced candidates this time around?
Most of it is just the combination of three election cycles, with Republicans down to only 40 Senators after the 2008 elections, and then quite a few of those retiring in 2010. By my count, that leaves only 33 Republican Senators who have been around longer than Scott Brown. Of those, Mitch McConnell and Jon Kyl were both born in 1942 and would therefore turn 70 in 2012, while nine others are even older. That's not an absolute disqualification, but it's certainly a serious problem for someone applying for a job who wants to hold it for eight years. That leaves 22. Of those, two are scandal-ridden. Two others are far too moderate to compete, while a third just lost a GOP primary.
That leaves all of 17 Republican Senators who will have served for at least four years, will be young enough, and are not otherwise disqualified. Most are obscure (Barrasso? Risch?) although of course even feinting a run at the White House is a way to become an awful lot less obscure. Some are unpopular with major party factions (Graham, Hutchison) or back in their home states (Corker, Burr). Several come from small states (Crapo, Johanns, Enzi, and of course Thune). It's not impossible to win a nomination if you come from a state with few people and little money, as George McGovern and Barry Goldwater proved, but it's harder.
Some may have reasons, good or bad, for not wanting national press attention to themselves and their families. It's even possible that a handful of them just aren't interested, although that's a bit harder to believe.
So: it's an interesting curiosity if it turns out there's no one out of the Senate in 2012, but I don't think it's a marker of anything. Just happened to work out that way. It's still the case that most Senators see a president when they look in the mirror, and that continues to be an important ingredient that helps make the Senate what it is.