I've seen the same beviour on the ActiveSocial Login module and with an XMod addin. To reproduce the behaviour, go here:
http://dnnprofessor.com/DNNDirectory.aspx
Type "hunting" in the Search For box and then hit Enter. Nothing happens. Now click on the Search link to the right... now you get search results. You can also click in the Search For box and then hit Tab, which will move focus to the Search link. Hitting Enter will now work properly, because the focus is in the right place
I expereinced the same problem with the ActiveSocial login module. After typing in my username, password and hitting enter, I would get a print modlue page containing the text of the module in the Content module. As it turns out, the print icon of the Content module was the first link on the page. I moved the login module to the LeftPane of the skin, which placed it ahead of the ContentPane. Now it works properly.
So my conclusion is that if there is no active link/button, hitting Enter triggers whatever the first link on the page happens to be, which may or may not be the desired effect. This is probably due to the fact that there's only one <form> tag, so how is DNN to know which action to take. However, it would be much better if hitting Enter triggered the first action following the control that had focus at the time.