Using custom type attribute in <script> tags such as jQuery's text/x-jquery-tmpl

I noticed that jquery's beta template plugin is using, the type attribute "text/x-jquery-tmpl"

e.g

<script type="text/x-jquery-tmpl">

I've not seen custom use of the type attribute in the past. Has anyone seen current examples of this in use or perhaps ways mere mortal developers such as I can use this in our own code?

I presume that it's sort of a MIME type, however I would of thought that MIME type support was up to the browser. So I would of assumed that custom MIME types would be unsupported?