Very weird -- that's one of the few features I didn't have any problems with. To ask some obvious questions (please don't be offended if you're very savvy and these were obvious to you as well):
Are you clicking "save" after you make the changes?
Did you have to do more than one install? Maybe it's posting to the wrong install.
When you go to the site to check on your changes, do you reload the page? Often browsers don't bother to reload a page if you've just been there.
Are you talking about the regular text on the page, or the big banner heading? That heading isn't in the "main page" and can't be edited by the define_pages editor. (Should be, that thing was a BEAR to get rid of -- wonder if it's still as hard on upcoming 2.0)