*Hello World* would look like Hello World in bold

_*Hello World*_ would look like Hello World in italics and bold

source: http://tech-buzz.net/2007/01/16/how-to-send-bold-or-italicized-messages-in-google-talk/

Also see:
http://blogoscoped.com/archive/2005-08-25-n18.html

thanks Tony! i missed it