@snood,
Now that Obama has given a green light to Biden to "consider" a run, I think there is an excellent chance he gets in. He'll have to contend with the Clintons and their machine, but he's a big boy and has a lot of good will and allies in the party. I'm very sure that if Obama asked him not to run, he wouldn't.
The talk now is that he is thinking running on a one term campaign, with Warren as VP. I doubt the people talking have inside information so it's probably just speculation that might make sense.
Warren is no spring chicken herself, and if she were to wait another 8 years to run, I believe she would be 74. Obviously her chance could come sooner if a Republican wins in 2016, but it's probably prudent to base decisions on the worse case scenario (for your goals).
If she runs with Biden and he bows out after 1 term, then she will have pretty close to a lock on the nomination for 2020. He could, of course, renege on any deal citing all kinds of reasons why the country needs him, but he doesn't seem like that kind of guy.
The problem with the one term campaign though is that if you win, you are something of a lame duck on day one. However, it can also be empowering in a lot of ways, so maybe it's a wash.
Timing means a lot in these matters. I don't think it's a coincidence that Obama gave him the green light after he spoke with Warren which suggests, to me, she gave him whatever assurances he was looking for.
On top of all this we don't know just how much Obama and Biden know about Servergate. You have to believe they know more than we do. If Clinton has a good chance of weathering the scandal, I don't think Biden would be in the news or that Obama would have given him the green light.
It is all speculation but it's fun for political junkies.
Clinton's campaign is so lackluster and she is such a crabby ass, that Smiling Joe Biden would be a refreshing entry.