Sitting canter without stirrups is fine.
Unfortunately, coming back to trot tends to be slightly more of an issue
Trouble is, if Joy feels me bouncing/likely to fall off she stops dead for me to sort myself out which is sweet, but not very helpful at times!
check out Enlightened Equitation's new youtube vids. There is one on sitting the canter.
http://www.youtube.com/user/enlighteneduk
also look at the Fred and Erik one, it is SO cute!
i personally find it much easier without stirrups than with them.
maybe its just me hehe