W3C recommends putting a space before the closing tag in XHTML, because this would give a better backwards compability with some browsers, e.g. write <br /> instead of <br/>. But are there still browsers out there, that would not tolerate that you omitted the space? (W3C do not mention which browsers cause problems.)
I know it doesn’t make much of a diffence. I just prefer the shorter version. So unless there is a good reason I will now start coding my XHTML without spaces before closing empty tags.
It’s for Netscape 4.
I still include it out of habit, and my templating library will put them in for me anyway, but it’s questionable whether it’s really that important today.