It seems that both on-line and in the news, about 95% of the Veep speculation circles around candidates who are currently running. In fact, WHO Monday seemed to think that Kerry-Edwards was almost a done deal. This surprises me because in my lifetime, the only time a primary opponent has become the VP nominee was Reagan-Bush in 1980. I'm not saying the names mentioned are unsuitable, just that the pool of possible candidates is large, and the final decision is usually an executive one by the nominee, not popular acclaim from the voters.
Some non-candidate possibilities?
Bill Richardson (he gets some mention here, but has already taken himself out)
Tom Daschle
Mary Landrieu
John Glenn
George Mitchell
Dick Durbin
Blanche Lincoln
Tom Vilsack
Janet Napolitano
Ben Nelson
Joseph Biden
Kent Conrad
This list could be expanded, and some people on here might have problems I just have thought of, beyond the concerns about holding Senate seats and so on which come up at the party level. I'm basically just brain-storming.
Another I've noticed...often VP choices are as much about elaborating on or balancing the ticket thematically as they are about given states or regions. Dole didn't pick Kemp for NY, but for youth and energy. Bush nabbed Cheney for gravitas. Gore wanted to show courage and a break with Clinton.
Just some thoughts.