It doesnt matter with WiFi what device you have, as long as you can access it and know credentials for logging onto it, unless its open as most places like Barnes and Noble McDonalds etc are now.
As far as the iPad and being on an AT&T specific WiFi like Wayport access, they know based on your NMD or SIM if you have an iPhone or iPad 3G or other AT&T mobile device.
Your question is open ended... WiFi is just that, nothing to do with AT&T unless its a secured network and you need an account, that is when AT&T would come in or whomever else owns the HotSpot. If you dont have credentials in the form of an account or username pass for that network, then you dont connect without pre paying most likely, however as I stated earlier most are open.
The iPad WiFi only unit to a hotspot, without having any 3G sim, looks to the hotspot like any other WiFi device. Even though AT&T gives free access to all iPhone users on any Wayport hotspot, the wifi only version of iPad will still look like a regular device. The hotspot, if running a web log in interface or other type of web based served applet, can look at your browser and see if its mobile safari or whatever else and serve that type of welcome page.
Not sure any of this answers you exactly, but I hope it helps.