Non-iPhone Data Plan

Hey there. I just wanted to know if it was possible to use a non-iPhone data plan with an iPhone 4. I know it was possible with a 3GS running iOS 3.x by changing the APN settings I believe. Can anyone confirm this still works on the iPhone 4? I mean I have have the sim card of a compatible phone that has a plan like "Data Unlimited" for $15/month : http://www.wireless.att.com/cell-phone-service/services/serviceDetails.jsp?LOSGId=7295504394&skuId=sku1160057&catId= or "Messaging & Data Unlimited" for $30/month : http://www.wireless.att.com/cell-phone-service/services/serviceDetails.jsp?LOSGId=7295504394&skuId=sku1160058&catId= or "Messaging 200 & Data Unlimited": http://www.wireless.att.com/cell-phone-service/services/serviceDetails.jsp?LOSGId=7295504394&skuId=sku4510225&catId= could I just cut the sim and put it in my iPhone 4 (or change the sim online to my new iPhone 4 microsim) and just change the APN settings so I wont get the "Could not activate cellular data network: you are not subscribed to a cellular data service" error?

Also, is this "Data Unlimited" package really unlimited or does it have a 5GB cap or something? Also can ATT or does ATT usually do anything about iPhones on a non-iPhone plan? I used to have a crappy samsung phone on a family plan with just voice ($9.99 for the extra line) and I simply put that sim on my iPhone 3GS and used it just fine for about 9 months or so... Thanks.

Edit: Ps: If anyone wants to pm me the info or their story instead of posting it publicly, please feel free to do so.

 

Because of the data plan or the voice plan without data? How do you know this? Did it happen to you? I used my old 3GS on a family plan with voice only for like 9 months without any problems...

 

Can anyone please shed more light on the subject? I'm very curious/interested. Thanks.

 

Bump? Please, I'm desperate to know if I can use a non-iPhone data plan... Thanks