Don't use the excuse that your Canon camera will match your Canon printer, that's just silly.
That being said, the XT is a nice entry level camera. It's pretty cheap now, so it's a good way to get into the world of the DSLR. It also means that if you get really into photography and want to pick up a prosumer body (40D, etc) then you don't lose as much money as if you went for a $1000 camera as your first purchase.
The XTi could be an option for you too, it has many advancements over the XT (but ignore 8 vs 10 MP) but also costs more. The XSi is ridiculous, and the new "features" aren't worth the higher price.
Take a look at Nikon too. While Nikon and Canon are comparable, generally speaking, the ergonomics and build quality of the entry level Nikon are arguably better. This difference disappears when you hit Nikon D80+ and Canon 40D+, but I believe it holds true for the XT/XTi/XSi and D40/x/60.
EDIT: I realised I didn't actually answer your questions.
People are divided on Lightroom vs. Aperture. Given the fact you have a MacBook, I'd say go with Lightroom. It runs a lot faster. I'm personally really bad at organising photos, and use files and folders to organise... which is bad.
As for XT vs. XTi.
8 MP vs 10MP
XTi has dust reduction
2.5" LCD on XTi, vs. 1.8" on XT
XTi has shooting info on the main LCD, rather than the XT's dedicated little LCD screen
$470 vs $600
As for lenses... I think you may as well go with the kit. It's not an awful lens, and the more you use it the more you'll realise exactly what more you'd want, whether that be telephoto, low light, wide angle, etc.
Some will say to look at Pentax/Olympus/Sony, etc. but I say don't bother. Stick with the "big two" for easy access to parts, accessories, repair etc.
Whatever you choose, research it, and have fun!