The question is, what does he do for four years? Run for Senate? Run again for Governor? Move to another state and run? Get a talk show? He needs to keep himself in the national spotlight as much as possible. That's clearly what Mike Huckabee is trying to do, speaking of another guy without an office.
The 2012 primary campaign will begin in two years, not four. Remember that virtually all of the 2008 candidates announced between December 2006 and February 2007. The first debates were in April and May of 2007. So, if Romney is planning to run for president in 2012, there's really no time (and no need, to be honest) for him to run for some other office in the interim. Most Americans have no clue who represents other states in the senate or in governorships anyway, so I'm unclear as to how Romney will be any less in the "national spotlight" than, say, Tim Pawlenty, even though the latter is currently in political office while the former isn't.