Both presidential campaigns are doing a very good job of keeping the progress of their vice presidential searches a secret. With respect to the most obvious question: who's on the short lists? -- answers won't be forthcoming for a while. Here's what I do know:
(1) Timing: the McCain campaign is looking at an early August date to reveal the choice; the Obama campaign is looking at early or mid-August; both campaigns are aware that the Olympics begins on August 8.
(2) Vetting -- having spoken with aides and advisers to folks I'm guessing will be on the short lists, neither candidate seems to be at the point where they've begun to formally vet a smaller selection of potential nominees. In general, only a handful of VP hopefuls on both sides are asked to submit to interviews with lawyers, to relinquish their medical records and tax returns, to alert the vetting teams to any potential scandal lurking in their pasts. It does not seem as if the major short list suspects have yet been contacted by the search teams.
(3) Winnowing -- My best guess is that both candidates are in the stage where they're reviewing open source political and personal data for any number of potential candidates and are in the process of figuring out whom they want their vice presidential search teams to approach.