Armstrong was the strongest rider in 2003, but Beloki attacked as soon as they hit the mountains. As soon as Armstrong chased him down, Vino would attack. The attacks forced Armstrong to chase far ahead of his team, effectively isolating him. The real threat to Armstrong that year is the possibillity of Vino/Beloki attacking, then Ullrich 'bridging up' to form a powerful lead group with Armstong's team not strong enough to follow.
Once Beloki crashed, only Vino had the legs to attack (Ullrich's not a natural attacker), so Armstrong could go one-on-one with him, where he was stronger.
This year the same situation rears its' head. Vino is back, and Basso proved last year that he can attack in the mountains as well. If the two attack alternately then Armstrong may be worn down, isolated in front. Basso may be all tuckered out from the Giro (like Simoni), but apparantly Iban Mayo and Roberto Heras have vowed revenge for last years' debacle. Throw in some unknown factors like Kloeden, or Leiphiemer, or even Spanish sensation Valverde, and you will have the most exiting Tour in many years.
Don't be surpised if Vino is the real team leader at T-Mobile. He has let slip more than once that the team expects him to attack in the mountains, and the 'party line' of him riding for Ullrich may be an elaborate deception.