According to the weight comparison chart found somewhere in the middle of this page:
http://www.dcresource.com/reviews/canon/digital_rebel_xti-review/index.shtml
The EOS 400D weighs 510g, while the Pentax K100D weighs 560g.
Reading your aforementioned statement, i would interpret it as "400D feels lighter than K100D because it's like a cheap plastic toy"
Does that mean anything below 560g feels plastic toy to you? if that's the case, the following camera would feel even more plastic to you as they are even lighter than the 400D.
Canon Digital Rebel XT (350D)
Olympus EVOLT E-500
Samsung Digimax GX-1S
How a camera feels can be affected by it's weight, but also by the materials used in it's construction. The D40 is the worst imho. 400D and 350D are next.