This turns out to be more complicated than just the amperage rating of the power adapter.
This XDA post says that the Tablet itself may 'step down' to 0.5A charging if it cannot detect the right support in the USB host. There are some additional details in this post but I don't grok them.
That thread led me to this slashdot comment which seems very useful and knowledgeable. If I understand the story correctly, unless the tablet can negotiate with the host device, the amount of power it can draw is limited. A physical "hack" (now part of a USB standard) in the adapter can make it clear to the USB client (the tablet) that there is no host device (its a wall wart), so it can charge faster. Without the hack, the device is limited in the amperage it can use.
More details on the USB charing spec are on wikipedia.
I just verified some of this with my Samsung Galaxy Tab 10.1 and the no-name 2A USB wall charger I have. If I plug my tablet into this "unapproved" USB adapter, then I get a little red "x" on the battery charge indicator in the system tray. Additionally, I get the "media scanner running..." alert (which I think means that the Tablet thinks it might be connected to a computer). I cannot tell at what rate (2.5mA, 500mA, 2A, etc) the tablet is actually charging at, though.
If I plug the tablet into the 'approved' wall wart, I get no such red "x", nor any "media scanner" alerts.
All that said, in practice, I've found that the tablet will charge up on my no-name USB adapter. I'm not sure if that's because I tend to turn the screen off (and I don't have 3G), and thus the power draw of the sleeping tablet is less than the 500mA, or if its actually getting the full 2A.
I think the upshot is that any reputable wall charger or anything labeled as "compatible" with an iPhone or iPad will probably support the USB charging spec, and thus will fast-charge a Samsung Galaxy Tab. But I'm just guessing. It would be nice if there was an official USB logo or label for chargers that supported this mode ...