Lots of things to consider here...
1) Aren't there some full frame SLR's that accept Nikon lenses- such as Sigma or Kodak?
2) If you are shooting digital now and you switch to full-frame, then all your lenses would suddenly "change" their field of view, i.e. all of your lenses just got to be 2/3 as long as they were.
3) Some people don't think you NEED full frame, such as...
http://kenrockwell.com/tech/dx.htm
Ken is certainly interesting to read and not shy about expressing his opinion. There's probably another side to consider so if a person is willing to spend the $$$ for full frame who am I to tell them they are wrong.
4) There's nothing bad to say against Canon. One thing good about Nikon is that if you elect to switch then all that big glass still has good resale value and good demand on the used market.