Regardless of whether it's a method="post" or method="get" in your form, if you submit a form using 'Enter' (the Enter key on your keyboard) vs. if you click the submit button to submit your form, you will get different values passed in the different browsers.
I'm lumping FireFox and Safari together on this one as they act the same, Internet Explorer 7 being the odd man out.
Firefox/Safari will pass the 'First' submit button along with your form (first in the DOM or on page) when you hit the 'Enter' button to submit the form (like in a simple search box etc.)
Internet Explorer on the other hand will NOT pass any submit buttons if you don't click on them explicitly.
This could be looked at as a good thing, in the event that you have multiple submit buttons and are doing different things based on the value of the submit button etc.
Regardless of which process I believe is correct or not, the Internet Explorer interpretation of this data was unexpected for me as I test in Firefox all the time. My solution was to simply put a hidden form field that is named the same as my submit button which I knew will get passed through the form regardless of what browser they are using. This fixed the problem for me (there are other solutions like disabling the 'enter' button to be pressed in a given box etc)
<input name="txtBox" value="" type="text">
<input name="otherstuff" value="x" type="hidden">
<input name="btnSubmit" value="Submit" onclick="return true;" type="submit">
<input name="btnSubmit" value="Submit" type="hidden">