Its not specifically AMD, although they never gurantee a newer CPU will work with a older socket/mobo/chipset. AM2 has the ability to support a Phenom based CPU, Abit just decided not to update their BIOS to support it.
^ Pretty much as mousemonkey said. They did update a few of their boards to support the first Phenoms. Since your board should technically support Phenom CPUs, you could possibly hack the BIOS to ad such support yourself. Of course, that requires you to delve into the world of BIOS hacking, where doing something wrong will land you with a dead board.