Unless you're working with XML files (or might be in the future), it's mot a problem. That's why I said "potential".
If the PHP parser ever comes across an XML declaration and short tags are enabled it will throw a nasty error message at you.
"string literal" or some such, I forgot.
Anyway, it's just another reason to not use short tags.
I'm not even sure why there is such a thing. To let programmers save a few characters? I don't think so. My guess is it's because other languages have similar syntax and PHP "copied" it.