ASIC's are not a natural technological generation leap like going from CPU to GPU was. ASIC's are simply specialized processing units made specifically for Bitcoin. Which i do not beleive is following the original intentions of Satoshi, for many reasons.
Satoshi was an intelligent man. He was well aware of the possibility of a hardware mining development arms race. Also one can make the following observation:
There was only
1 GPU chip manufacturer who dominated mining farms: AMD.
Today there are
2 FPGA chip manufacturers who are at the basis of all FPGA farms: Altera (in the BFL Single), and Xilinx (Spartan6-LX150 in all others).
And soon there will be
3 vendors of ASIC mining chips: BFL, Tom's, and nghzang's companies.
So we are actually becoming
more decentralized over time.
IMHO the number of owners of the IP (intellectual property) of the technologies at the core of Bitcoin mining is a better indicator of how resilient (which is what you mean by decentralized I think) Bitcoin mining can be, assuming hypothetical scenarios where hostile parties would try to take control of, or negatively affect, this technology. It is not the number of resellers or distributors that matters (like you point out there are many AMD graphics cards resellers).