IE7 is still in beta but with things apparently being layout complete, this hack may stick around until final launch. It’s a variation on the underscore hack.
(This code snippet is from http://snook.ca/archives/html_and_css/targetting_ie7)
Here’s a quick example:
border:1px solid green; /* works in all */
*border:1px solid red; /* targets IE6 and 7 */
_border:1px solid blue; /* targets IE6 */
Yes, this is invalid CSS and it might get fixed by the time IE7 hits the streets.