I wish I could dock my iPhone right into the side or back of a large iPad screen. And then have the iPad run off my phone. That way I could have a large screen when I need it. But when I leave the house for a walk, I could take the smaller device (the iPhone) out and carry it in my pocket.
Love and snuggles,
Your nephew Gary
Please excuse Auntie if she comes across all cranky but that lumbago is acting up -- and lumbago can be a real pain in the, well, ... you know. Auntie isn't entirely sure what you're trying to get at. If you own both an iPad and an iPhone, you can sync both devices to the same computer and share bookmarks, contacts lists, email accounts, and so forth. You can read the same mail, visit the same Web spaces, and play the same games on both devices, so why duplicate the screen?
Admittedly, it would be lovely if iTunes allowed more shared preferences (including application file management and app folders), but even so, moving from your iPhone to the iPad's native interface doesn't take a lot of work, and there are lots of lovely applications that leverage the cloud so that your data and interaction experience remains consistent across devices.
Now, before you get all "please answer the question I actually asked" on Auntie, let Auntie answer the question you actually asked. Yes, you can do this already, so long as your devices are on the same Wi-Fi network. They don't even have to be on a real network together, an ad-hoc one will do just fine -- via a spare Mac laptop, for example.
You'll need to jailbreak your iPhone and install Veency, a free application from Cydia creator Jay Freeman. Veency provides a VNC server that runs natively on your iPhone, providing a way for you to share your iPhone's screen with other devices. Start a VNC session from your iPad (using a client like iTeleport or iSSH), and voila, you'll have your iPhone screen running on your iPad. Veency could use a little iOS 4.x love, so this "solution" is even more hacky than you might first imagine.
I'm not entirely sure why you'd want to do this but, well, there you have it.
P.S. To expand your screen in general, you can always hook up a jailbroken iPhone to a TV using Ryan Petrich's superb DisplayOut enhancement.