In my previous life as an ASP.NET ninja, I had a request from a client to use the enter button to submit a form. It turns out to be a little bit tricky until you find the magic code. Today I was the same question and found the answer after a quick Live Search.

EmailTextBox.Attributes.Add("onKeyPress", "javascript:if (event.keyCode == 13) __doPostBack('" + LoginButton.UniqueID + "','')");

This adds some javascript to your textbox which performs a postback on a specific button when the enter key is pressed in the text box. If you're interested in the ASPNET2.0 way of doing this, check out Scott Guthrie's comment on that post.