According to a Marc Ambinder blog this afternoon -

The selection of ex-VA Gov. Mark Warner to deliver the keynote address at the Democratic National Convention comes on the heels of a secret, last-minute effort to convince Warner to submit his name and record for vice presidential vetting.

Sources close to Warner say that the Virginia Senate candidate was subject to fairly intense pressure by Obama advisers to allow the team of Eric Holder and Caroline Kennedy to open an account and begin their work.

Warner resisted, citing, as he has done publicly, family concerns and his public pledge to Virginians.

Full blog available here - CLICK ME

Many have said he would have been the best pick, and it was hard to argue with that sentiment. Alas it wasn't meant to be.  By giving Warner the valuable keynote speaker spot, he's basically sending the message that Warner is his pick in 2016, given Warners age and credentials. This also means that it is unlikely that Obama is picking an younger VP, because in most instances that person would be Obama's defacto pick to replace him at the end of his second term. 

I understand why Warner made his decision, but Obama/Warner would have been a helluva ticket.  Warner has the stability that he wants for his young family by winning the Virginia senate seat, and could reasonably keep it as long as he wanted it. In four(dread) or eight years time he could run for President as his own man and off of his own record.   If he ran with Obama and they lost, Warner would effectively be locked out of Virginia politics with Webb, Kaine and whomever won the vacant senate seat there for the next while.  Also he probably doesn't want his young family growing up in the White House spotlight.