No surprise there.
In many ways, Joe Lieberman is exactly that McCain wants in a VP candidate. More than anything else, Lieberman is a loyalist who McCain can count on to carry his water. Lieberman also has national name recognition that Tim Pawlenty, Bobby Jindal, and Sarah Palin don't have. He has experience in a national campaign from the 2000 and 2004 campaigns. Finally, Lieberman is very smooth with the mainstream media.
It's easy to envision McCain using Lieberman in an unorthodox way. Instead of Joe campaigning on his own like other VP candidates, McCain could have Lieberman travel with him and serve as chief surrogate, minder, and friend all at once. Like Richard II, McCain "need[s] friends" and needs to have them by his side. McCain tends to get off track with his campaign principles and talking points. But he can count on Lieberman to stick with the McCain line even if he himself can't.
The only reason McCain hesitates with Lieberman is that Joe is a Democrat whose liberal social views (pro-choice on abortion, etc.) would set off a firestorm among social conservatives.
But it's not as if McCain either likes or respects social conservatives.
So, it could be Joe.