I understand you dilemma. I currently own a 10D, and while I love the camera, I need to get up to megapixel speed, so to speak. I'm looking at the XTi, or taking the big plunge for the 5D. You might laugh at the camera choice, but I face the common problem of need and desire for the better camera, but $$ for the cheaper one. (I don't think the megapixel jump from the 10D to the 30D is worth it. There are probably other factors to consider, though.)
I've handled the XTi and I find the body to be like the cheap 35mm cameras used to be, that is to say, cheap. I don't like the grip, though it's a minor fator, I don't like the double button setup for aperture adjustment, and I don't like the smaller viewfinder (much smaller than the 30D, the D80). But it's affordable, and I imagine the image quality is not that far off from the 5D (I have yet to make comparisons over at DP Review, however).
Anyway, back to your question of 10D vs XTi: The build is definitely better with the 10D, but, if you're like me, you don't bang cameras around. The kind of shooting I do, and I am a professional, is mild mannered, often in the studio. The body will hold up just fine for me. The ergonomics are another thing, though. The smaller viewfinder is a hindrance, especially for accurate alignment in the composition (though I have yet to shoot with an XTi). And, I hate to admit it, my eyes are getting older (not the rest of me, no, no