This is actually a global Apple cut behaviour, it's thinking of the domain as a word to be inserted at paste time (because you double clicked it). I run into this problem all the time using the forum when copy pasting URLs. Although we also wish the Apple way were to paste what was selected and not an extra space, it's more important that our programs behave the Mac way (not to mention removing the smarts from copy/cut/paste is tricky.
Sounds easy but there is a lot of sharing of a single NSTextVIew for editing in order to save memory).
What is interesting is that the cut/copy/paste is smart enough to take into account a trailing space and make that part of the paste and cut, but not smart enough to realize there is no space prefixing the domain but a period and therefore the word should not be prefixed with a space (URLs are not even mentioned in their
word selection guidelines). John Grubber has an interesting
write up about it. As you will find from the article dragging to select avoids the space; so by not double clicking the domain but swiping it you get just the domain – but that might be more work than removing the space.
Apple bug database is not public but there must be reports asking for the smart copy/cut/paste to be smarter or better yet not smart at all. Let's hope this is the case in OS X 10.6 (small domestic house cat).
cause it doesn't seem like there is a reason for it to be implemented there. Unless it's a global code thing.