Play.Com seem to be the first major retailer with prices for Rock Band on the Xbox 360. The good news is they also seem to be way below the RRP of £179.98 at £139.98. Add on voucher experience08 for 5% off to bring the price down to £132.98 and go through quidco for another 4%.
Edit - There seems to be some confusion over this so let's see if we can clear it up here:
1) The instrument pack does NOT include a copy of the game. This is stupid beyond belief but that's the way EA decided to play it. If you order the instrument set on its own @ £99.99 you won't get the Rock Band game.
2) At the moment all Guitar Hero controllers for the 360 seem to work with Rock Band and vice versa, or at least they did for the US release. On the PS3 the controllers for both games are not compatible (note, some queries over whether GH controllers work in RB).
3) Rock Band is a timed 360 exclusive in Europe. In the US it is available for 360, PS3 and PS2 with a Wii version on the way. Europe will get these versions "later this summer".
Note this is for pre-order, the game comes out on the 23rd of May.