I think the major reason why ebooks manufactures don't incorporate backlit screen is because of how much backlit screen decreases battery life... Plus if this device is to mimic a traditional book, books don't have lights built in ;-).
The solution is buying a reading light as you've always have for traditional books and use it with your e-book reader. Here are some examples from the Sony readers:
My wife does a lot of reading at night and we have purchased just about every book reading light ever made. I would like to get her an ebook reader that has a backlit screen. Does it exist?