Been googling for answers and have found nothing, then stumbled on this crazy simple fix.
A bunch of my sites' RSS feeds were having a problem / error not validating when I tried to check them at http://feedvalidator.org/
I would get this validation error:
line 1, column 1: Blank line before XML declaration
I tried deactivating my plugins - that didn't work.
I tried switching themes to default TwentyTwelve - that worked, so it had to be something in my theme.
Turns out it was a very simple problem with my functions.php file in my theme...
There were extra spaces at the end of the functions.php file after the closing php ?> tag.
I just removed all the spaces before and after the closing php tag and PRESTO - all is good everywhere.
Hope this helps some people.