A WiFi sled (is there one for the Treo?) will not use the Cingular network.
In order to access the Net via WiFi you will need access to either a
wireless access point (such as a wireless router on your home network) or a
WiFi Hotspot. Not all WiFi hot spots are free and may require a one time
(daily) cost or a monthly subscription. In my opinion if you're going to pay
a monthly fee for Hotspot access why not just get a Cingular data plan
instead? WiFi will only be useable when you are near an access point whereas
a Cingular data plan will allow net access almost anywhere you can get a
cellular signal.

Cheers
TC