IE actually considers this one a feature (hold on, give 'em a chance). See this article explaining why this isn't necessarily a bad thing.

If you really want IE to submit the form, there are a couple work-arounds. One nice work-around is that, IE will submit the form on ENTER if you include in the form a hidden input field with the same name as the submit input.

<input type="hidden" name="find" />
<input type="submit" name="find" value="Find" />