[Update 2011-12-20-120457: see end of post]
I sometimes get issues with files that come in from another source, that have the BOM – usually from some other web developer who’s using Windows.
It’s not a massive issue, but often introduces space that you can’t see except in the browser where it can break some designs.
One problem is that vim and most (all?) graphical editors don’t show it to you so you can remove it, so you usually have to shove it through perl or something. Another method that’s extremely easy to use is to edit the file in nano. I’m not a massive fan of nano, but what it gives you in this case is the ability to see the BOM as what looks like a single space at the start of the file, so you can delete it very easily. Handy.
There’s an even easier way, just in vim; not sure what from version this became available.
Check to see if you have a BOM:
Get rid of it: