Well, the most popular business use of Airstreams seems to be mobile food service businesses. I guess there is a cupcake bakery in Texas that operates a fleet of Airstream cupcake shops. Here are a couple of photos of the Surreal Gourmet trailer from the 2004 Airstream Homecoming Rally.
If the prototype for the HO-gauge model Airstream in this post over on the "Sex in an Airstream" thread actually exists (which seems fairly doubtful) then that would certainly be a colorful Airstream-based business!
Also at one of the Homecoming rallies was an Airstream set up as a mobile prostate cancer screening center. Rally-goers (of the male persuasion) were invited to drop in for a free digital rectal examination
. I don't think there were too many takers.