I happened to have a test case appear. A brand new Blackberry Bold 9000, from AT&T. No SIM, no activation of any sort, was able to connect and surf via WiFi. The little "WiFi" indicator stayed dim, didn't go bright like it normally does with a data plan enabled SIM inserted.
With a previously enabled SIM/cellular based BIS authorization, the same thing works... you can surf, but you have to select the "HotSpot Browser" in the browser options screen.