The default button on enter on an ASP.NET form
EmailTextBox.Attributes.Add("onKeyPress", "javascript:if (event.keyCode == 13) __doPostBack('" + LoginButton.UniqueID + "','')");
"EmailTextBox" is as you might suspect a text box control, while "LoginButton" is a link button. All it does is render an extra attribute in the rendered tag that checks for a key press, and if it's the enter key, it fires ASP.NET's __doPostBack method with the unique ID of the button that you want to virtually press. That in turn fires off whatever server-side event handlers you've wired up. You could pass in the eventargument as the second parameter in that Javascript if you wanted to.
I can't understand why this wasn't included somewhere in v2. They did offer a default button for the entire form, but I think that only works if you have actual buttons (I use LinkButtons almost exclusively). You could actually wire up any control name here if you wanted.