Wired has an excellent article for Apple's (and mainly AT&T's) reasoning for switching SIM card sizes in the Apple iPad. The reason? Money, plain and simple.
The iPhone uses a full sized SIM card and is much smaller then the iPad. So size constraints aren't the reason.
It's quite easy to see that by not allowing you an easy way to switch your SIM card between devices, AT&T can charge you for both the iPhone as well as your iPad and thus increase revenue.
Don't be too upset however as you can modify your existing SIM to fit into the iPad by cutting it down to size, however you will need to buy a converter to then re-upsize it to fit into your iPhone. So all of this wrangling just once again makes it harder on the customer.
More info at the Wired article here: Apple iPad’s Tiny SIM Is Just There to Mess With You | Epicenter | Wired.com