Without question- John Warner is Virginia's most popular elected official (followed closely by Mark Warner). It'll be interesting to see what John Warner does in the 2008 battle between Mark Warner and Jim Gilmore. I can't imagine he endorses Delegate "Sideshow Bob" Marshall in the convention, so the question is does he endorse either Jim Gilmore or Mark Warner?
If written reports are to be trusted- Mark Warner and John Warner grew very close during Mark Warner's tenure as Governor and I've also heard that John Warner doesn't particularly care for Jim Gilmore. So John Warner has three options in 2008:
1) John Warner endorses Jim Gilmore after the convention. Given the personal animosity between Gilmore and Warner as well as their idealogical differences- a Warner endorsement could give Gilmore credibility with moderate voters and shore up his support among the Republican base. That said, voters could also just write it off as a Republican endorsement of another Republican (as they did when Warner endorsed Kilgore and Allen).
2) John Warner doesn't endorse anyone at all. This would be a media circus for the Gilmore campaign- with headlines like "Republican Maverick Withholds Support from Fellow Republican" and articles talking about Gilmore's perceived unelectability and partisanship (exactly what a campaign wants to be talking about :-P).
3) John Warner endorses Mark Warner. Governor Warner is already perceived as Senator Warner's natural ideological successor- so this endorsement would just cement that idea in the minds of the voters. However, a John Warner endorsement of Mark Warner would further fuel the idea of an ideologically transcendent Virginia Democratic Party and a fractured Virginia Republican Primary.
To be clear: No matter what happens- I still think Mark Warner will be the favorite in the race- I just thought it would be fun to explore the possibilities.
Kaine's down right now, but John Warner is only at 60/31 presently. Mark Warner was in the high 60's/low 70's when he left office.