Well, the RIM Playbook (yet to be released) must be tethered to a Blackberry device in order to access email, calendar, and address book.

However, I don’t know if that tethering will also provide Internet access. RIM is planning to release two versions, one just with WiFi and the other with 3G cellular + WiFi.

I wouldn’t be surprised if tethering for Internet is technologically possible… but if so, it might not be available due to carrier restrictions (à la AT&T initially not allowing iPhone users to tether and then later requiring a special data plan).