When Internet Explorers AutoComplete is turned on for Forms the entries for each field in the HTML form should be cached and displayed as a prompt when the user starts enter
dgavey - how are your forms being submitted? Microsoft isn't overly clear about this subtle issue, but;
YOU HAVE TO SUBMIT YOUR FORMS WITH A SUBMIT BUTTON
for this to work!
Unless of course you are like every site out there that likes to design their own forms thank you very much, and thus you'll need this hack to make IE behave.
http://webbugtrack.blogspot.com/2007/08/bug-137-ie-autocomplete-hardly-ever.html
which points the KB article here, where MS explains this really odd behavior.