The BOM Signature is enabled for this page.
see this thread:
http://www.webassist.com/forums/posts.php?id=19285
and the last section:
2) The Unicode Signature (BOM) PHP bug. the forth screen shot shows the Title / Encoding category of the Page Properties screen (Modify - > Page Properties). Checking the Include Signature (BOM) option can cause the headers already sent error