Zubielda and Mayo obviously have a lot of incentive to win tommorrow since the race passes briefly through the Basque country (I believe) but unless they have an absolutely incredable day I don't see them as long term threats (they would have to put 6 minutes into Ullrich or Armstrong to have enough time to keep the yellow through the TT on friday.
Vinokourov is a completely different story. I was suprised to see Ullrich and Armstrong let him go without a fight, but even with a similar performance tommorrow he'd have a tough time keeping yellow through the TT. But if he could have an above average performance tommorrow then who knows. I definitely think he's got a podium spot and I can see him possibly beating Armstrong or Ullrich for second but I really don't think he has what it takes to beat both of them.
Tactically I think Armstrong has done pretty well. Lots of people including Liggett said they thought Armstrong should have given the jersey to Ullrich so that he would have to defend it. Instead by sending Beltran up the road he made Ullrich's team ride at the front of the peleton the entire day anyway and didn't have to give up his 15 seconds.
Having said all that I think that if Lance doesn't have what it takes to attack tommorrow then he'll lose the tour to Ullrich in the final time trial.