Are there any security issues with using a tablet (for Rick travel guides!) inside museums?
We used ours in the Sistine Chapel on our Friday night tour. We had the entire place to ourselves and spent 45 minutes in the Sistine Chapel with maybe 15 other people in there. Others gathered around us to hear as Steve went through each ceiling panel on our Kindle Fire. We used all of his travel guides we could.
I think you'll see lots of other travelers with them and also using their photo function (which I find awkward with a "full-sized" iPad).
I do not think that you will have a problem, but since most tablets have multiple functions, if the museum says no flash, no flash; and if they say no pictures...no pictures.
Check the website for the museums you will be visiting. More and more of them have free apps you can install on smartphones/tablets which replace the audio guides they rent out.