First of all, I would like to disclose that I am a total amateur. I am a tech geek however, and try to stay on top of technology as much as I can, even in the photography field.
Anyway, I've owned a D60 with two kit lenses for a little over one year. I really wanted the Canon XSi but loved how the D60 felt in my hand, it seemed "more substantial and better built" at least to me. The Canon just felt a bit more "plasticy" if I may say so. Now I use a professional photographer for my events at work 3 times a week, and talk to him all the time, he uses Canon 50D and takes amazing pictures. So nothing against Canons, I do like them.
However, between the cameras that you are comparing, I would definetely go with the D60. For one reason or another, the darn camera takes better pictures than the XSi and the XS... and I can say that for sure, at least in the hands of an amateur. Other family memebers have the other two cameras, and I have numerous pictures that I've taken during family events with all three cameras, at the same position, same light, same settings, and the D60 pictures blow the other two away, to the point of me having to e-mail my set of pictures to the rest of the family because theirs did not turn as good. Colors seem more saturated, better exposed, the XSi seems to wash out the pics a lot. Now, I am sure that in the hands of a professional, the XSi and the T1i should be able to take better pics than the Nikon D60... but in my hands, in the hands of my friends and family, I can GUARANTEE you better pictures 90% of the time with the D60... Don't ask me why, I've looked through the specs a million times, and the XSi and T1i look much better on paper... until I take the pictures with them! Crazy!
To be honest, the Canon T1i is on sale at Costco with two lenses for $899 at the moment... great deal! I've been itching to buy it, and was on my way out of the house two days ago to go get it (money is not an isssue keep in mind) and my wife wouldn't let me - she LOVES the pictures the D60 takes, and is aware of the difference with our friends/family taken pictures with the XSi and wouldn't want to hear it. Basically she said I can buy it, but the camera that will be taking pictures this Xmas will be the D60, and that would be the one that we will take on all vacations and etc... And I wouldn't listen to her if I also didn't feel the same deep inside, I just love new gadgets so I'm very tempted! But also very afraid, because I highly doubt that I'd be able to take the amazing pictures that I take with my D60 with any other camera, or at least not without having to splurge on nice lens and countless hours of learning how to use it professionaly. I get compliments every time I post pictures on facebook or anywhere else, people constantly ask me what camera I'm using and etc. and half of the time I'm shooting in fully auto mode! Even my professional photographer who I mentioned earlier talks to me about some of my pictures and comments on how amazing they look.
Anyway, just my .02 cents in this conversation, from someone that loves technology and really wants to believe that 15 megapixles is better than 10 megapixles - sometimes its just not about the specs or the numbers...