Hab dazu was gefunden (Englisch), das Problem hatte ich damals schon mal im IE6. Anscheinend gibt es das oder ein ähnliches Problem im IE7 auch.

Fun with multiple submit buttons by Stuart Colville

1. Variations when using Enter to submit in IE

I remembered that my colleague Tim had mentioned a while back that he had experienced an issue with IE not sending the name/value pair for the submit button when Enter was pressed after filling in a field in the form.

Tim knocked up a quick test script so that we could see what happens with IE. After testing we found that IE doesn’t send the name/value pair for the submit button if you press enter without focussing on the submit button first. However after some further testing of my own I found that this behaviour only happens if one text input is used. If you use two or more text inputs, the name/value pair of the submit button highest in the source is sent (more on that below). From my tests this seems to be the same for IE 5-6.0 and IE 5.2 Mac.