To be honest, the 7D does generate a fair bit of noise at anything over ISO 400, but no worse than any other modern DSLR in the same price bracket that I come across in my day to day. It will be certainly better than the 400D. But as Beau said it's just best to try avoid shooting at high ISO's where possible. Overall the 7D is a great camera and if you are forced to shoot at 800 or 1600, a bit of post processing in photoshop and you can still print a decent 6x4 out of them.
Here are some examples of the noise, I've just cropped these out of the originals, so there is no resizing at work here to hide the grain. The blurriness is just down to a plain old lack of talent
ISO1600
ISO800
ISO400
Hope that helps Nige.