Of course there's an app for this, but I'd never even considered it. While I do occasionally read .cbr (Comic Book Archive files) on my desktop, the iPhone didn't really occur to me as a portable comic book reading device. But sure enough, there are apps for that. iComic is probably the cheapest, but I hear it's a little tough to get set up. ComicZeal is extremely popular, and just a little bit cheaper from the somewhat similar myComics. Pull Lists uses a desktop client to sync up comics for you, which is a little more than I need, but it seems like some people like it. There are actually lots of choices, it turns out -- I would have thought that the iPhone's screen was pretty small for reading comic book pages, but I'll have to give those apps a try.
Or, if you're too cheap to pay a buck or two for a full app, you can even do it yourself. .cbr files are really just .jpg archives, so as this tip from a while back on the iPhone Comic Book Reader says, you can actually extract out the jpegs, and then just sync them up to your iPhone via iPhoto if you want. Three dollars really isn't that much, and any of those apps listed above will give you a little more functionality (easy flipping between pages, bookmarking, and many other features per app), but if you really want to DIY, there you go.
Now it would be nice to be able to officially buy .cbr archives over the web from some of the major companies -- the closest they've come that I know of is a subscription service, but of course you've got to use their own client for that (and there's no iPhone app that I can find, though Uclick has a few apps in there for certain books). As long as reading pirated .cbr files is outlawed, only outlaws will end up reading them.