I think there have been some pretty high sales of nnnnn.com. It mostly depends of the city they represent. Los Angeles ZIP code will get you much more than some small town zip code.
Think about the business. The domain is highly unlikely too be used by a professional company and is not rememberable. People forget how many N's to put. I'd say it is worth less than reg free if you want me to be honest.
I'd agree with jasonsc, some ZIPs are more valuable than others but I believe all ZIPs are taken. ZIPS are definitely worth more than reg fee.
Zips are on a bit of a roll now, but I just can't see any real value in them. A very famous zip might be useful for a real estate agent, but there are few zips that are that well known. In the UK, London postal codes like NW10, on the other hand, are often referred to. They are all taken and have some value. But zips? I can't even remember my own. If I had a zip, I would sell it for what I could get for it.