Got it. I think it was Berry Sprinter, a Sprinter dealer from the phone conversation. They were totally cool about it- when I realized the 800# was a dealer I imagined at the least a heavy sigh from being asked about yet another radio code, but the guy didn't have any issue punching up the VIN and giving me the code.
Berry also has a published PDF, but it doesn't go as far as a late-year 2006 VIN (apparently), so for us it was the phone number that did it.
http://www.berrysprinter.com/radio%20code.pdf
This topic is now about how stupid it is to have a unique 5 digit PIN for a tape deck which is also (I'm told) printed on the back of the very radio unit itself.