Well, I hate to break it to you, but the Canon competitor to the D300 (The 7D) is just as long in the tooth and is also way overdue for replacement. Jumping the fence won't solve the issue.
I am somewhat curious what you feel you are "missing" in the lineup Nikon currently has.
>At this point I see no reason to believe that Nikon has any >plans to expand the pro or pro-sumer DX line. > >And at this point I don't know what to do about it. I need to >upgrade from my D200 but Nikon has nothing that would serve as >a reasonable upgrade to a D200 level body given that the D300, >though very capable, is quite long in the tooth already. > >Not sure what Canon has to offer. I'm such a long time, die >hard Nikon guy I can hardly bear to look. But that is not >going to last too much longer.