That's a load of BS (what the sales people told you.)
You do NOT need a data plan to use the phone portion of the Treo on the AT&T or Cingular network. Point of fact, when I signed up they did not activate my data plan (as they were supposed to) so for the first day I was stuck with voice only and no data until I got them to add my data plan.
Yes, if you buy an unlocked treo from PalmOne directly and just put your SIM in, it will simply just work as a phone with no data access. If you do have data access it's because they have a data "pay per use" enabled on your account. You can have them remove this if you like so you don't accidentally get charged if you accidentally go online.
If you get an AT&T subsidy locked phone from AT&T wireless, it will only take an AT&T SIM. You'll still be on the Cingular network as they merge the two, but it won't be a native Cingular SIM. I don't know if there is any reason that's bad, as far as I know they can treat your AT&T SIM as a Cingular SIM from the network and billing side if they want, but I'm not sure.
One way around this is to get the AT&T phone. Demand the sibsidy unlock code some time in the future if you ever need/want the Cingular SIM but you have to be careful because AT&T is known for NEVER giving out unlock codes. Cingular does have a policy of unlocking them after a while and since they are merging, I don't know how that affects the old AT&T policy. Does it follow the AT&T phone? I don't know.
Per the whole data plan thing. I recommend at least getting the $10 Cingular MEdia Basic bundle
https://www.cingular.com/media/media_net_purchaseThis gives you a very very small handful of SMS, MMS, and data.
But if you want to save $10, that's your call. At least this way you get some very limited use of the addtional messaging and data features for a low price.
But $10 is $10, if you don't want any of that, you do not need it. The treo is simply a phone like any other and can work with just the voice plans.
WHen I got my Treo 600 a long time aga on AT&T, they considered it a data device with a phone attached (Primary data, secondary phone) vs. the other way around like most phones. I think they still have this perspective.
For SMS and MMS, you don't need a data plan, you only need a voice plan. MMS and SMS (multimedia and text messaging) is part of the voice network and without a multimedia or text messaging package they will simply charge you per message. MMS and SMS "pay per use" is enabled by default. But again it may be more cost effective to get a MEdia Net bundle.