Bruyneel has worked out a tactic which plays to Armstrong's strengths -- or rather, to one strength in particular. Armstrong has a higher anaerobic capacity than most of his rivals, which means that he can pour on the gas for a couple of minutes at a time. On a flat road, the aerodynamic advantage of riding second in line is so great that even if Lance did this, most riders could keep up. Going uphill, speeds are reduced, and aerodynamic advantage is less. So if he puts on a spurt of speed in the mountains, he can ride away from anyone else. Once he's away from the other riders and the spurt is over he's not particularly faster than the other top contenders, so one useful tactic is to do this near the end of a steep mountain climb at the end of the day. This saves him from many miles of riding alone at the head of the stage while his rivals work together to try to catch him.
OK, nice tactic, but it's only good for a minute or two worth of advantage. And there may not be a suitably steep section the right distance from the end of the stage (in fact, the Tour organizers have gone to some length to make sure there is not, to make it possible for someone else to win). What do you do when the steep section is further from the end, perhaps not on the final climb at all? The key is that it's not as simple as saying that no one can keep up with a surge from Lance. Other riders can, but doing so tires them out so much that they won't be able to keep up later in the race. In fact, other riders can even produce a surge of that magnitude, though it will cost them the next day or (if it's not at the end of the stage) later on that day. So on the first day in the mountains, Bruyneel had some of Lance's teammates do exactly that. Lance could keep up, because he's Lance. Lance's teammates could keep up because they didn't have to think about the next day. Once they'd dropped their serious rivals, the other Discovery riders could pace Lance part way to the end of the stage, helping him gain more time. Of course maintaining that pace was too much for them to keep up with, so they could only take him part way, and once they were gone he didn't gain any more time. But he didn't lose time either, and by attacking early he could pick the right section of road and also ensure that anyone following him would pay the price. In fact, a young rider named Allesandro Valverde did follow him all the way to the end, but then got sick due to the impact of the effort on his immune system and dropped out a few days later.
Hold on, though. This was the first day in the mountains; why didn't Lance's teammates have to worry about the next day? The next day, the T-Mobile team tried a tactic that was suited to their particular strengths. With three strong climbers and a downhill finish, they prefer to attack on hills that are not as steep, going faster for a few minutes than Lance can maintain but short of his surge speed, and forcing him to use up his surges to respond. By taking turns with this kind of attack, they can get a rider off the front, and the downhill finish makes it harder for Lance to catch up later. If Lance has his teammates with him, he can have them respond while he follows in their slipstream, and he doesn't have to use up a surge. But he didn't have any teammates with him, because they were still too tired from the previous day to keep up with the elite riders. And even on days when his teammates aren't tired, if there are too many attacks they might not be able to respond to them all. One of the T-Mobile riders gained some time this way; why wasn't it more, and why didn't it happen again?
And the answer here is that if Lance had not already made his move and gained time on the other riders, it might have been more of a problem. But because he's had his team go all out on the first day in the mountains, he not only gained a couple of minutes on his chief rivals, but he also gained about half an hour on everyone else. Now there are lots of riders who aren't a threat to Lance, and who might try to break away on a flat part of the stage. Lance doesn't care if someone who was 30 minutes behind gets off the front of the pack by 25 minutes. But the riders 5 minutes behind Lance care, because there is money for second and third place and the riders in the breakaway might bump them off the podium. So their teams spend a lot of effort trying to keep the breakaways from getting too far ahead, and Discovery only has to exert itself if the other teams can't keep it in control. With several contenders for a podium finish, T-Mobile spends a lot of effort in the early part of the stage, which leaves them less able to attack him in the mountains later in the day. In this way, it's to Lance's advantage for someone else to win the stage. But that's only true if he's already made his most powerful move, and that's why, for 7 years in a row, that move has always come at the very first mountain finish of the Tour.
What's really wild is that, since there is also money for winning stages, if a rider knows he's not going to be a threat for the overall lead, it is to his advantage to lose time to Lance early in the Tour, so that he won't be chased down if he tries to break away on the flats. This applies to climbers as well. Mickael Rasmussen took it easy in the first time trial, and doing so paid off handsomely when he led a day-long breakaway over several mountain passes, collecting lots of points toward the polka-dot jersey of the best climber. Had he done better in the time trial, Discovery would have given chase sooner and he might not have been first over all those climbs.