This accessory is not made to work with iPhone

Whenever I plug my iPhone 3G into accessories that I used previously with my old iPod -- including an Altec Lansing iM600 external speaker system and a JVC car stereo -- I get the scary message "This accessory is not made to work with iPhone" (followed by a message about switching to airplane mode). That message is almost mind-blowing scary like "Your iPhone is going to be fried if you stay plugged in to this device". Is it really that bad?

After poking around for a while I've found a few links that seem to be helpful. Rather than repeat what they have to say, here are the best links I've found:

These links report that the biggest problem is cell phone audio interference, but I don't know if that's the full extent of the problem. I haven't found anything that says my iPhone is going to be fried, and indeed I've used mine for a few hours now with these devices, but I've got to say user beware, check with your manufacturer, etc. Nobody wants a fried iPhone.