I found a discussion of possible problems, the writer seems not to like the idea, though doesn't rule it out completely.
Issues that are addressed there (my comments in parentheses):
- Holding something valuable during Davenning is prohibited since the possibility of it falling bothers כוונה. (A Siddur is permitted, I woudn't assume a difference since people who are davenning are as careful not to drop a Siddur as a iPad)
- Jumping alerts can interfere. (This seems to be a real problem, my phone can block messages when an app is up, but not phone calls. A device which can shut off everything during davenning, would not have this problem.)
- One should be able to Daven without distractions, including looking for things in a siddur, so he should have page numbers all written down. (Still, a siddur is better than nothing, searching a PDF for הלל wouldn't be different than flipping pages, IMHO. Moreover, Siddur apps give you the relevant davenning only, and in order.)
- While davenning one should hold his hands by his heart (Again, I can't see why an iPad would differ from a Siddur here)
To sum it up, I personally don't see a problem. I would add one should have a regular siddur for use on Shabbat and Chagim, because if he gets used to relying on the iPad he can get stuck on those occasions.