If you want the ultimate, then buy a Jawbone bluetooth headset. It uses military grade noise cancelling technology.
The demo on the web site has someone talking on a phone with a leaf blower in the background. The person on the other end of the conversation cannot hear anything but the callers voice! Another neat thing is that it automatically increases/decreases the earpiece volume in relation to the external noise level.
It's not cheap but it does work as advertised! You can buy them locally at a AT&T/Cingular store or order them online from different retailers. There are also lots of reviews on-line, for this product too.
http://www.jawbone.com/ (be sure and check out the demo)