Sure, it's too early, but Marc Ambinder floats Mark Warner on the Democratic side and Mike Huckabee for the GOP. Both are from the South, which makes a certain sense, given that this is likely to be the first Presidential election since 1988 when neither candidate hailed from Dixie - or since 1984, if you count George H.W. Bush as a Texan. I'm pretty skeptical, though, of the notion that "regional balance" is a particularly important value for a campaign to seek these days. It made more sense in an era when political parties were uneasy, ideologically-diverse coalitions, and when machines could really deliver a state to a candidate: Picking Lyndon Baines Johnson as his running mate, for instance, pretty clearly helped put JFK over the top in '60, whether you believe there was fraud in Texas or not. Nowadays, though, the parties are more ideologically coherent, and regional loyalties are more attenuated; hence John Edwards' failure to help John Kerry in the South in '04, and Al Gore's failure to carry his home state four years earlier.
It seems to me that what you're looking for these days is a nominee who gives you a narrative that the media can embrace, more than one who gives you a slight boost in a swing state or region. Thus Gore was a good pick for Clinton in '92, even though they were both Southerners, because he reinforced the whole "New Democrats, new generation, new direction" theme that the press ran with throughout that election. Similarly, I think that Dick Cheney and Joe Lieberman helped their respective tickets in '00, even though neither Wyoming nor Connecticut mattered to the outcome: Cheney by lending necessary gravitas to a campaign premised on restoring honor, dignity and so forth; Lieberman both by being the first Jewish nominee and by having a reputation as a Clinton critic, which at once turned the dull-seeming Gore into a trailblazer and helped him distance himself from the Clinton scandals. I know Democrats soured on Lieberman in '00 because of his debate performance and what they perceived as his weakness during the recount, but picking him gave Gore an enormous boost that August, and helped the Democrats close what had been an impressive Bush lead in the polls.
Obviously there's no way of knowing how things will play out this time around. But here's one example of what I mean: If Barack Obama wins the Democratic nomination, he has an interest in picking a Southern running mate (like Mark Warner, say) less because the pick might help Obama carry some Southern states than because the narrative that such a pick projects - a black candidate with a white running mate from the old Confederacy! - dovetails perfectly with Obama's "beyond our differences" appeal.