My guess is that very soon, you will need a real account to buy comics in Comixology. Which is actually a good thing, because I've had pretty good luck with Amazon's online payment system buying other goods through them.

I also think Amazon--as a service to iOS users--may be working on a touch-centric Comixology web site that works under Safari for iOS. That way, you get something close to the in-app purchase of the old Comixology app, but you launch this web site first in Safari, buy the digital comic(s), then it will automatically load into the Comixology app.