A little late to the party, but I've got the M series of these: http://earplugstore.stores.yahoo.net/chmisearph.html

I figured the price was about the same as the race shops were charging, but they can serve double-duty with my iPod whenever I want. I did have to pick up a stereo --> mono adapter to use them with the in-car radios (which are all wired for mono sound). $5 or so at RadioShack.

They worked perfectly with my radio setup, as well as the not-my-radio setup in the enduro car.

Jarrod