You're really not going to notice dust unless you stop down to f/11 or smaller. I only notice dust when stopping down to f/16 or smaller, and even then it's small. And I change lenses all the time, and I've never cleaned it. I tend to shoot at f/1.4-f/2.8 so dust isn't a problem for me.
A Giotto Rocket blower will get the job done, most of the time. If not, there's some Copperhill thing (uh hopefully someone has more info on this, I don't know much) that will clean it too. I think it involves brushes and liquid so I'm afraid to try it. That's just me though, the Copperhill way works really well.
Maybe a camera store would clean it, they'd charge you though. I think Canon does it if you send the camera in for something, they might do it as a service too. Btw, I hear from the folks over at DPreview.com that the intergrated cleaner in the 400D doesn't work all that well.
Dust is inevitable. Using zoom lenses will suck air into the camera and cause dust to go inside, changing lenses will, etc. The camera comes with dust in the sensor right out of the box, usually. It's annoying, but I feel that it's an overblown issue with people going crazy. "How come my brand new camera has dust! OMG, I've stopped down to f/22 and I see specks, this camera is defective!"
