Whilst I may have a bit of a wait for a digital version of Pluto or 20th Century Boys (the Manga I am reading voraciously at the moment), there are options available already to access some Manga on the iPad. NTT Solmare have been publishing individual mangas as apps on the iPhone for some time, and have now brought out their first iPad HD release, Always By Your Side/Solaruru. Their books are translated into English for the UK/US market, and anglicised so that the books read left to right, rather than the traditional right to left.
I’ve tried out this first release (it was only 50p, their usual price seems to have been about £1.79), and having read it, it’s a very simple tale of a ghost appearing in a school, and befriending a girl. There isn’t much to the story, I wouldn’t recommend it. And the translation is a little clunky, the text seems to have been placed in rather large font onto the page with little care. However as they’ve published 120 iPhone mangas, there is the potential they could publish something interesting to the English-speaking market in future. So I would suggest having a look at the iPhone/iPad stores, and searching for “NTT Solmare” to see if anything takes your fancy there, if like me you’re looking out for digital manga.