I'm sure by this point I must do this too, without realising it most of the time. Actually, I know I do, because now and then, I see something I've written and think...tsk.
I've been using US English and spelling when writing for so long now, mostly because it's usually been US shows I've written fanfic for, that it tends to all blend together by this stage in the game.
LabRat (who was brought up short the other day when she used the US pronounciation for garage while talking to Stuart
"The car's in the garage. Garage?!? Huh? What? Eeesh.")