I don't think anythings impossible when it comes to rfid, there's a few exciting things going on over at (http://www.openbeacon.org/) for example, unfortunately, passive rfid has a few limitations. Like, collision detection is possible using 13.56, but there is a limit on the number of objects that can be simultaneously interrogated.
Also, 13.56 still only has an effective read range of about 6", depending on the size of the tag and a few other variables. I think there's a few creative solutions being used by casino's for chip interrogation, so you could google that to determine if there's anything suitable for the application you've got in mind.
You could also start with the ASPX RW210, there is code around Arduino for that somewhere, and it's a step up on the parallax which is one of the lowest of low-powered 125khz readers, I did the parallax "thing" as well and it was quite disappointing.