I got the Canon 500 f4 a couple of month ago and love it, I'm sure you'll really enjoy using it. The 300 f4 IS is apparently a very good lens too (though I've only used one briefly), I almost ordered one this week as a walkabout lens, but have gone for the 400 f5.6 instead as I always want more reach for birds. I guess a nice replacement for your 18-135 would be the 24-105 f4 IS, it's a great lens, but it doesn't come cheap.
I've never had Nikon kit so cannot comment on the change, I came to Canon from Minolta (an excellent move). Andy Bright made the Nikon to Canon switch earlier this year and he seems very pleased with it.
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