Codenamed Thuban, the chip will squeeze all six cores onto a single 45nm die. Thuban is derived from the hexa-core Opteron released earlier this year, and will feature an integrated DDR3 controller. It will likely have lower clock frequencies than AMD's current quad core parts, is expected to house 3MB of L2 and 6MB of L3 cache, and be pushed to market as a Phenom II X6 -- but those tidbits aren't official.
Based on the Opteron's specs, MaxiumPC speculates that Thuban will be a 346mm2 chip with a massive 904 million transistors. For reference, Intel's Core i7 975 Extreme Edition has 731 million transistors on a 262mm2 die, and the Phenom II X4 965 Black Edition 758 million on a 258mm2 die.
AMD's hexa-core desktop CPU won't debut until sometime next year, and probably won't arrive before Intel's Gulftown.