Not really, the 400D is a great camera, I remember mine fondly. If you don;t need video and are happy with its performance then yeah keep using it. The new cameras are better, things like live view, and usable higher ISO's are great, wireless flash is really useful as is the flip out screen. The new cameras will do almost everything a little better than the 400D, but that doesn't make the 400D a bad camera.
You might actually be better buying a Canon 40mm f2.8 pancake. More compact package, camera you know and love using, and if / when the 400 dies the 40 will work on whatever you buy in the future, canon wise at least.
Best of all, the 40mm is cheap!