The example here is oil. Oil is traded with USD only (even though that is starting to change now). Since you can only buy with USD, that creates a built in demand for USD.
I would say the killer app in the case of BTC would be something which requires anonymous financial transactions due to any number of reasons. (Not just the illegal ones)
A product that could only be purchased by BTC would be great for the currency, but would it be good for the product? Unless, as you mentioned, it would be something in which anonymous purchase was integral or an advantage already, it could cost the product maker a lot of sales by restricting its market.
One hazy idea I have is patent (or other IP) insurance. Patent applications can be expensive and time-consuming, and insurance that someone else doesn't beat you to it may not be ideal if you don't want to reveal that you have something. I can see engineers wanting something like this if they were worried about lawsuits with their employers about IP rights.