I Don´t want pay large cost for Roaming.

dou you know? www.padinthecity.com

They have the service of Rental Ipad with Unlimited and Fully internet access and Madrid Apps. (cheap, no extra fees, no roaming, low cost).

Buy a normal sized SIM and trim it to fit. People are already showing video of it using TMobile SIMs cut to size in the USA. 3FF SIMs have the same electrical contacts and follow the same standards as normal SIMs, just they have less unused space around the chip.

Also don't be surprised if 3FF sized SIMs start to become available for other devices and phones. Jobs didn't pull the form factor out of thin air, it's a standard that was developed by the ETSI

...where can one buy a pre-pay Micro SIM for an iPad?

Given that TMobile will charge you a mere £7.50 ($11.50) per megabyte on a pre-pay SIM to roam in the USA (and most of the rest of the world) I wouldn't try holding them up as a shining example.

My original point holds true: as the Pad is contract free and unlocked then you're better off buying a local pre-pay SIM and using that.

Not all data roaming is this extortionate - at least not in Europe. Recently on holiday and needed to use my android phone a lot to get updates on volcanos etc! Needed to buy a couple of 20MB european roaming data bundles from Tmobile at £5 (~$7.50) each - in retrospect I should have bought a single 50MB bundle at £10. Not "cheap" but not extortianate compared to the ATT rates ... but then again, I buy 6 months of "unlimited" (ok ~1GB/month fair usage limit) internet access for my phone for only £20 (thats £20/$30 for the entire 6 months and not per month!).

your service provider is, they charge you an arm and a leg for data roaming. The iPhone allows you to disable data roaming. I've no doubt that the iPad will also allow it. The iPad is however unlocked. You can pick up a SIM from a local provider and get your data from them direct at local rates.