I've just downloaded onto a new iPad with retina display, IOS 6.

The book selection using grid mode works fine in portrait. If you rotate into landscape the last 3 "boxes" on the right hand margin are missing, and the page won't scroll all the way down to the bottom of the screen. I can get it to work by rotating into portrait, selecting the book, then rotating back again. I can email you a screenshot to show you what I mean.

until it's fixed. I'll have to use the list mode, which works fine.

I've found the problem, the above bug is only a symptom. I went into the options page and noticed that the "allow rotation" flag was set to off. If I set it to On, then it all works fine. So it would appear that this flag is getting ignored somehow, as mantis is certainly rotating into landscape mode when I turn the ipad.