IMHO, DC17 Animal has a flawed design with the sealed canister (at least as purchased in December 2006 purpotedly has been corrected at this time).
The rubber gasket (where you close the bin at the bottom) is so delicate, that even the routine maintenance of emptying the bin can dislodge it (or even crimp it) so that debris (carpet lint, pet hair, etc) enters the cyclone itself eventually clogging it.
Same experience as Mark A with customer service, but with persistence in explaining their faulty design to them (particularly with regard to the fact that it's caused by routine maintenance), they'll break down and send you a new cylone unit - from there on out ensure that the thing is sealed.
Although the original question is somewhat dated, hopefully this will save others some aggravation in dealing with the problem.
Grumpy old guy that doesn't take no for an answer...