Tethering your iPad with your iPhone requires an additional device which is used to establish the ad-hoc network. For example you can use a Mac for this, as explained here: once your Mac is connected to the iPhone you can add the iPad to the ad-hoc network.
Once the connection has been established, in theory the additional device (f.e. the Mac) is not needed anymore and your iPhone and iPad are going to maintain the connection by themselves.
This is only in theory. In practice the connection is going to die in a few minutes, for whatever reason, and you will be unable to reconnect. This makes using this connection unpractical, as you would not only need to carry the iPhone and the iPad, but also the Mac.
The solution I have found, which I still find more acceptable than paying an extra mobile subscription just to browse the net on my iPad while on the go, is to use an additional device: a portable Wifi router.
(Actually I cannot even pay my mobile provider to activate the tethering option in the iPhone: they refuse to do it. I would have to purchase a separate device and subscribe to an additional plan!!!)
The portable router will create a small network that will allow your iPhone and your iPad to communicate and will maintain this connection and re-establish it if it is broken for whatever reason. Optionally, it will allow you to share the connection with someone else if you want (using another iPad, Mac or even a PC).
I use iProxy 2.0-39 (commit 188599ca7c with some changes to make it compilable under Xcode 4.3.1) together with a Trendnet TEW-655BR3G portable Wifi router. You should be able to find it for less then $75. Any other portable Wifi router should work anyway. The requirements are:
I purchased the Trendnet because it was available at a nearby store. It turns out that it has a very good battery life: around 5 to 6 hours! It's much longer than advertised (2h30), probably because it is normally used with a 3G key, which it has to power. I do not have to plug a 3G key because the iPhone is the link to internet, so I'm getting more battery time from the router. I have also configured it to disable what is not needed, which helps.
It is provided with both an AC and a car charger, which is great. Please note that it is not always provided with both, so check this out before purchasing.
Here is how to configure the 3 devices (the iPhone, the router and the iPad):
At that point, the iPad will be able to use the iPhone's 3G connection to surf the web with Safari. It will not be able to send or receive emails, or iMessages. I don't care much, I can do these directly on the iPhone. Or I can use GMail by its web access. What I really wanted was to use the iPad to read web pages on the larger screen.
I have however noticed that I could browse the App Store and even download/install apps on the iPad when working under this setup!
If you have problems, try this: