<p>Where do the type/member/parameter descriptions that you see in the Intellisense bubble come from? Are they stored in type attributes?</p>

<p>EDIT: I'm specifically interested in the built-in types.</p>

I don't know about the built-in types, but when you're building your own classes you can use Xml comments to list out the intellisense information.  The comments are put into their own xml file when you build the project, and as long as that file is in the right place Visual Studio will be able to use it.  Not only that, but you'll get intellisense help to build the comments.  For C#, just add an extra / character to a comment line right above a class, property, or method: ///.  For VB, use two extra ' characters: '''
One caveat with this (and I'm sure there's an option somewhere to control it), but it's been my experience that once you put an xml comment
anywhere
in a project, you'll get a compile warning for
every
public item that isn't commented if you don't use them
everywhere
in the project.