Echo the "try before you buy". Also stress the unique characteristics of each system - lens range, flash features etc.
Canon has a wide range of consumer zooms, but their standout lenses for me are the 10-22 EF-S, 35/1.4L 50/1.4 USM, 85/1.2L, 70-200/4L. None of those lenses are cheap, though they're usually a bit cheaper than Nikon's alternatives. The 18-55 IS that ships with the kit is pretty decent these days (my dad has the old 18-55 kit which is a POS). If you're interested in dabbling with video, the 550D (Rebel T2i) is probably the most advanced video DSLR in its range.
I'm a bit less familiar with Nikon's range. Their 18-55 has been generally quite well regarded, and they have a good reputation in primes. I do know that many Nikon shooters prefer the Tamron 17-50/2.8 as their walkabout lens for a fraction of the price of the Nikon version. I have a friend with a D90 and the 10-24, 18-55, 55-200, 35/1.8m 18-105, 80-200, 105/2 (phew!) and he's pretty happy with it, although he also suggests you look at the 550D just because it is the most up-to-date midrange DSLR on the market at the moment.
I'm a Micro Four Thirds user myself, came from Canon L glass and pro bodies, but got sick of the weight. There are some really good lenses, but they are expensive and selection is still poor. However, the camera that you carry with you will always take better pictures than the camera you leave at home, so I have no regrets. My GF1+20/1.7+7-14+Legacy SMC-Tak 50/1.4 weighs about the same as my dad's 40D+50/1.4. There's a lot of good Oly glass on FT (not MFT), but my Panasonic body won't work with most of them. Shame
I do still feel some envy for the Pentax K-x, which is one of the smallest DSLRs on the market, and their range of amazing primes (I'm a prime man, a segment not too well served by MFT). The DA 15/4 Limited, DA 21/3.2 AL Limited, DA 35/2.8 Limited, FA 31/1.8 Limited, FA 43/1.9 Limited, FA 77/1.8 Limited are all lustworthy for me. Hm... come to think of it, maybe it's a good thing I didn't buy Pentax after all, since I'd be bankrupt in a week
. The K-x does have an annoying omission in that the active AF point doesn't light up, and that can be a deal killer for some.
Wind, Sand and Stars.