I think he meant that the USD/bitUSD ratio oscillates around a ratio of 1:1. However, this doesn't factor in the volatility. If today you get 1.10:1 and tomorrow its .90:1, that's a huge spread. It also doesn't imply that the average ratio is 1:1 - you need to calculate the price over time so see where its average is. Even though most of the us here know the difference between the peg vs the spread, its a tricky thing to explain to outsiders. Going with what toast has started, I think you can measure the effectiveness of the peg as following:
1. USD/bitUSD ratio that oscillates around a 1:1 ratio
2. An average price of $1.00 over any given slice of time (within reasonable min & max bounds...that the traders holding period fits inside)
If these conditions are met, the variance of the 1:1 ratio can be attributed to the spread - the size of which is determined by the liquidity of the market. Then what toast said about market making is true. I think he missed my bolded point.