I'm spending a couple of consecutive weekends in the U.S., so it seemed like the right time to equip my iPad with an AT&T microSIM card. Picked one up at an AT&T store in western New York free of charge -- interesting, since another AT&T store in the area said they'd charge $15 for one.
Popped the SIM into my iPad before leaving for a 6-hour drive, but never could scare up the AT&T network page (nor see any indication that the 'Pad was picking up a 3G signal.) Spent the first half of the trip on the phone with both AT&T and Apple support (while my husband was driving, of course.) This included explaining to at least three incredulous AT&T employees that my iPad was not locked to a Canadian carrier. (Duh.)
Finally brought everything to another AT&T store in northern Connecticut, where one of the salespeople finally realized that the first AT&T store had given me an iPhone card by mistake. They said they normally charge $15 for an iPad card, nothing for an iPhone card, and can't tell the two apart except by one or two digits from one of the three identification numbers on the front of the card. There's no visible difference -- they're the same color, and neither is marked "For iPad" or "For iPhone."
And, no, they didn't charge me. They put the correct SIM card in my iPad, and I have 3G service in the U.S. now.
It all worked out, but if you're trying to get an AT&T microSIM for an iPad, be aware that you may have to train the staff first. :-)