The effect I’m looking for is demonstrated at:
This is easy enough to do with Bing Maps 6.3 by adding a description to a pin, however this functionality seems to have been removed in version 7.0. Version 7 has a fairly nice info-box mechanism which I’ve employed successfully, but the info-boxes it creates are added as children of the map div, not the body element like in the example. Since the map-div has the CSS property overflow: hidden the info-boxes get cut off by the edge of the map.
Example: http://jsfiddle.net/FnWLT/2/
The simple, but time consuming answer is to create your own infobox I recommend this as a starting point from which you can customize the behaviour to your requirements:
http://bingmapsv7modules.codeplex.com/wikipage?title=Custom%20Infobox%20Control