I don't think anyone's mentioned this yet but the Marvel iPad comics reader is a white-label version of the free Comics by Comixology iPad/iPhone reader - which has about 2,000 comics on it from Marvel, Dark Horse and many more.
http://www.comixology.com/digital/
Their system is really wonderful; I use it for the iPhone and iPad versions of my wireless comic Valentine:
http://www.comixology.com/digital/94...tine-English-1