iphone 2gb + tethering and ipad

My understanding is the same as yours. That is you can use the phone as a 3G hotspot so you can connect to the internet using the phone's 3G with a wireless connection between the iPad (or maybe another device too). As you said, you have to pay a little extra for tethering plus there is no longer an unlimited data plan so you pay by the GB.

No, the iPad won't be able to tether with the iPhone(unless you jailbreak).

Per the following AT&T web page/FAQ tethering is now available for the iPhone via an additional monthly charge and a cap on monthly data usage.

http://www.wireless.att.com/answer-center/main.jsp?t=solutionTab&ft=searchTab&ps =solutionPanels&locale=enUS&dyncharset=UTF-8&solutionId=KB110271&isSrch=Yes

This is what it says if the links do not come out right:

"DataPro
Our DataPro plan is $25 per month and includes 2 GB of data (the equivalent of downloading 1,000 sixty-second YouTube videos every month). This plan is designed for our customers who regularly download or stream music and video, or use other high bandwidth applications. Additionally, smartphone customers who choose DataPro will have the option to add tethering for $20 per month. This is available to iPhone customers as well.
"

Does anyone know if there's possible to get internet access on ipad via bluetooth from a windows mobile cellphone (tethering)

The only solutions that I am aware of that get you Cellular Data from a non 3G iPad, are MiFi Capable devices from folks like Verizon and Sprint. Palm also has a phone that can share its 3G via WiFi (Palm Pre), and they have not been charging a tethering fee. These plans are typically more expensive at approx $60 per month for 5 GB.

The iPad Bluetooth is not configured to allow tethering to, or from, the device.