They'll never get anywhere close to Amazon. Amazon is the greatest of all giants as a merchant and is pretty much untouchable regardless of wether they accept Bitcoin or not. If Bitcoin keeps growing to where it is large enough it's probably only a matter of time before Amazon starts accepting them though. I'm sure the benefits will present themselves clearly over time and they'll regret not getting in sooner. I bet they're watching Overstocks acceptance of them quite closely.
I don't think that Amazon is untouchable. Overstock revenues are growing at around 30% per year, while the Amazon revenues are growing more slowly. It is just like the Yahoo vs Google battle during the 2000-2003 period.