I really wish Nikon made this lens: a well-built, wide, cheap (ish) zoom. You sacrifice some speed and separation control (f/4 vs. f/2.8), but it's lighter and cheaper than the pro version. The closest think Nikon makes is about $500 more and weighs a ton.
Because none of Canon's serious bodies have a built-in flash, if you plan on spending any time shooting in harsh daylight you're going to need to pick up a Speedlite. The 580 isn't as good as Nikon's SB-900, but flash is a trade off for going with Canon gear.
PocketWizard MiniTT1 Transmitter: Canon shooters have been lucky enough to use PocketWizard's new flash triggering system (wireless TTL!) for a while now. PocketWizard is still silent on when the Nikon version will be available.
Canon 24-105mm f/4L zoom lens: This is another great example of Canon making a really useful, high quality zoom at a decent price range. 24-105 covers the most common range you'd need (on a full frame camera anyway). Nikon has a low(ish) cost version of this lens, but it's not a great lens.