It looks like AT&T is making it easy to buy just one sim. You get a full size sim with the option for a micro sim. Once you punch out the micro sim though there is no going back.
If you decide you want to take your micro sim out of your iPad 3G and put it into your iPhone you can alway use tape to attach the full size sim.
These are not sold in the stores yet, but I am guessing we will see them tomorrow with the release of the iPad 3G.
Just got one Micro SIM adapter from here, works good so far: