A client came to me the other day complaining that she couldn't get the full RSS feed to work despite having the "full text" option selected in the settings. I thought this was strange so I went and tested it out myself on the most recent release of wp on a variety of themes including twentyten.
She was indeed correct, I couldn't get the full text of an article to show in any readers (Google reader, feedburner etc) and after some searching I found a fix here: http://nnbox.ca/2010/03/hack-fix-for-wordpress-2-9-not-displaying-full-text-rss/
The fix fixes what looks like a pretty obvious and glaring problem with the core wordpress code: A simple if statement is set to display the excerpt whether or not the full text is selected.
Although the link page refers to wordpress 2.9 I found the problem to still be there in my wordpress 3.1 install.
I'm amazed this hasn't been fixed and I someone could submit this to the "things to be fixed" place for the developers that would be great.