The VIZ Manga app for the iPad has been out since November, but they're currently having an introductory sale on all Vol. 1s (first issue in all their series) for $1.
The underlying comic-reading software isn't based on ComiXology, like Marvel's app is, but looks similar and works pretty well. On the iPad 2, page transitions are smooth and dialogue bubble text is readable—an important consideration for any comic app.
And prices are pretty decent! For example, Amazon has hardcopy Naruto volumes for $8, but they're just $5 on VIZ's app. Still somewhat pricey if you're going to go on a reading binge, but there aren't a lot of other outlets (other than quasi-legal/probably illegal fan translations that you can find online) for Japanese manga.
There aren't a lot of issues available in the app, on the whole, though maybe if they see a lot of adoption VIZ will dump more in. What's also quirky is that the app doesn't know or tell you when you come to a double splash two-page spread, so you don't know that you need to tilt your iPad to landscape to see the scene in its entirety.
The app is free, but you'll have to pay $5 for each book after you exhaust the handful of free samples. [iTunes]