short answer is no, it does not centralize bitcoin. it redistributes wealth to a few, but a few is an oligopoly at best. and oligopolies are still technically decentralized.
think of it this way: just because big banks hold billions or trillions of dollars, it does not centralize the currency in that bank. the banks are decentralized and competing against one another, and everyone else that has dollars at all. any little small business, or individual citizen. basically, it works like this:
1. someone makes lots of asics. in this case, avalon.
2. USD/EUR rich people become asic rich.
3. USD/EUR rich people become BTC rich people.
companies like avalon cannot consistently sustain massive profitability, and the bitcoin difficulty will skyrocket, making the competition harder, and further decreasing demand for expensive ASIC mining machines. to maintain profitability as an asic manufacturer you will need to make cheaper machines, which is possible. you just make smaller ones.
4. product segmentation occurs to capture more market share and value, 1 ASIC boards (e.g. jalapeno) come out at accessible prices.
5. USD/EUR poor people buy asics.
6. USD/EUR poor people get relatively richer in BTC, but not as fast as the guys that had the ASICs first because of hashrate->difficulty changes.
7. ASIC poor people (ie, people on GPUs) get f*cked completely at this point.
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an aside: look at all the exchanging going on here.
USD/EUR->BTC->asic
BTC->asic
gpu/fpga->BTC->asic
gpu/fpga->USD/EUR->asic
gpu/fpga->EUR/USD->BTC->asic
gpu/fpga->BTC->EUR/USD->asic
note each of these has different, fluctuating exchange rates in and of themselves. the math is not as simple as have asic? more usd.
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what you are confusing here is wealth distribution vs. centralization. everyone can still take part in the supply of bitcoins, even if there are ASIC guys. you don't need mining hardware to get a btc now, you can get one from mt gox by trading other commodities. some guy that has massive amounts of coins from GPU mining in 2010 is still powerful enough to make fluctuations on btc exchanges, especially because btc is FAR less liquid than other currencies.
a centralized currency is one in which one party can fundamentally control its flow. there is no central bank of bitcoin, no planning committee. what we may see is a heavy disparity of income distribution (economists call this a gini coefficient), but most likely bitcoins will fall into something pareto-like in distribution (80% of btc wealth is held by 20% of the population). but that's just distribution. it's not centralized.
the ASIC by itself will probably not cause centralization. a monopoly is required for centralization, and there is no monopoly on ASIC right now, and likely will not be. if someone sees a market opening they will build ASICs, just like the FPGA guys did and you had 10-20 vendors. you and 1000 other people could start an asic manufacturing cooperative if you wanted to, and build your own asics. nobody owns the exclusive right to asics. some people will just get richer first.