I've stumbled upon an interesting phenomenon...
If I copy any text from this page:
http://www.toyzdollz.com/fisherprice/boxplaysets.htmI can't paste it into the body of an email in Mail, whether it's a plain text or Rich email.
I can however paste the text into the subject or other header field. I haven't ever noticed this on any other web site's content.
I'm using Safari 4 beta (5528.16) and Mail 3.5 - I'd be curious if anyone else with Mac OS would be able to test this out and let me know what version of Safari/Mail you're using.