Mike Kleiman has been experimenting with the Hover CSS tags and presented his results over on his blog. This allows for a mouse over on a link or image where you can you expand the area and show additional text beneath it.

Mike’s test results also show some very promising results, although he does caution for Gmail recipients:

I know right away this won’t work in Gmail, since hover cannot be applied
inline and Gmail cuts out the style block. But it’s still worth an
experimentation anyways since I found Gmail simply defaulted to show the entire
content and so even when it breaks the user is still getting essentially the
full experience.

Please share your results if you test this.