Yes, you will lose your plan. The unlimited plan is not for tethering. If you attempt to tether the iPad to a stock iPhone, AT&T will see it as your request to switch over to a new plan. You will lose the unlimited plan and AT&T will start you on the tethering plan (for more money at that).
If you choose to jailbreak your iPhone, you could get one of the tethering tweaks that allow you to continue to use your unlimited plan and tether - without having to go through AT&T. However, do be aware that AT&T is always on the look-out for tetherers and just because you install the tweak, it does not mean that AT&T will not notice that you are tethering. So, this is a "do at your own risk" thing...
Hope this helps.
Marilyn
