I don't ever remember seeing this problem before.. Using the latest Evolution (1.4.5), all updates applied to my FC1 system, and now it's butchering my sig :( I've tried spaces, tabs and no difference.. Looked at it in vi, and it lines up ok... E-mailed another account where I use Sylpheed, and it looks ok... The line under my name gets moved to the left.. Well, at least when I receive it back, and look at the e-mail.. Any ideas?
I don't use Evolution, but could it be that you're composing using a non-fixed width font or possibly html?
If you're using html, try creating an html .sig (or switching to ASCII).