Mine have been sticky for years, but I always forget to research the solution. But recently I had the same condition occur with a spatula in the kitchen. It's the OXO brand and I wrote them because I have a number of their kitchen tools that share the rubbery grips. They replaced the spatula, but had no idea what the condition or solution was. Hard to believe, eh?
So, tonight I looked up this thread. Thanks guys, for the easy answers. Tomorrow I'll try the alcohol on the knobs and handle.
Will report back to the thread.

Here are new knobs..
http://stores.ebay.com/beckerautosound/Radio-Knobs-/_i.html?_fsub=5215146010

Thanks, but OUCH! Those are expensive. I'll salvage the old ones. I guess nobody ever said that owning a Porsche was cheap.

Well, the rubbing alcohol did a great job cleaning that sticky crap off the radio knobs. So, no need to buy new ones or plasticote.
Strange that the knob material and color are perfect.....so why was that original troublesome coating applied at the factory???

My Boxster has a radio?