Considering the prospects of Obama seating a young liberal who might be around for decades, conservatives would probably jump at the chance to support Specter. Republicans probably can't block an Obama nominee, but for public relations purposes, it's always good to have a smooth confirmation -- and Republicans would see Specter as the best choice they option they could hope for.
And what would Barack Obama get out of it? More than you think ...
First, he could honestly say he picked a highly-qualified and experienced (aside from being a U.S. Senator, Specter is a lawyer and former District Attorney) leader who is -- by definition -- bi-partisan.
Specter would also be confirmed without a fight, which would allow Obama to expend energy on other fights, such as health care. Specter's likely smooth confirmation might make him even more desirable as a second choice -- if Obama's first nominee were to run into problems (just as Harriet Miers did).
A Specter appointment would also clear the way for a real Democrat to be appointed by Governor Ed Rendell. (Remember, Arlen Specter is an iconoclast and cannot be counted on as a reliable liberal vote. Having a reliable liberal in his seat might come in handy when it comes time to fill the next vacated Supreme Court seat.)
Perhaps most important reason Obama should consider this is to avoid angering Democratic Senators -- who have already voiced concern that Specter -- who outranks almost everybody in terms of tenure -- might hurt their chances of chairing committees. As The Hill reported this week,
"I won't be happy if I don't get to chair something because of Arlen Specter," said Sen. Barbara Mikulski (D-Md.), who sits on the Appropriations Committee with Specter and is fifth in seniority among Democrats ...Lastly, picking Arlen Specter allows Obama to avoid offending any one identity group within the liberal coalition. Picking a woman might anger some liberal Hispanics. Picking a Hispanic might anger some African-American liberals. A Specter selection would allow Obama to avoid picking and choosing between liberal identity groups, all under the guise of making a statesmanlike bipartisan selection.
Arlen Specter for Supreme Court Justice. ... Not as absurd an idea as you thought, huh?