This clip of Bon Jovi's Wanted Dead or Alive gives you a good look at the HUD. Your two guitars are to the left and right of the screen (it doesn't matter which guitar is where) the drums are in the middle and the vocals are up top.
I'm really surprised at how well Harmonix was able to cram all four instruments on one screen and still make it look good. As soon as I picked up the guitar (or the drumsticks) it just felt natural looking at it. Granted, some people on really small TVs are going to feel a little cramped, but for the most part, I don't think anyone is going to have an issue.
I apologize for the singing in the background and lack of audio here. There were 3 different stations on the bus, so everyone wore headphones. This means that if you weren't playing, you got to hear the off-key singing of everyone in addition to the banging on the drums and the clicking of keys.
You can see on the left how well each person is doing. The bar represents how well the band is doing as a whole, and each logo represents the different person. As individual members move down, and eventually hit the bottom, they will fail out of the song. While they're failed out, the whole meter begins to move down quickly. Each individual member can be saved by using Overdrive (the equivalent of Star Power) no more than three times. It's really cool how the game depends on all four people working together to complete the song.
Sadly, this wraps up my coverage of the Rock Band Tour. It was an awesome experience, you should definitely check it out if it comes to your area. Thanks to John and Fabio for hanging out with me, and to the Harmonix team that's been working so hard on this game. All I have to say is that November can't come soon enough.