It is a problem that sometimes occurs - basically the sensor has been placed slightly out of alignment, so it is obviously recording at a slightly angle compared to the viewfinder (which is set straight)
A friend has this problem with his 350d, but since it is so far out of warranty now he just learned to live with it and compensate - either by just shooting slightly squint in the viewfinder, or purposely shooting slightly wider and cropping/rotating to the desired shot.
|Common Flashwing by digimania|
from canon dslr
|The Marilyn impersonator by Lee8282|
from Blowing in the Wind (Nature)