Brenna has an insightful post on why typographical errors aren't always the author's fault... even typos in the first line of the book:-). (All Brenna's posts are insightful; if you haven't checked out her blog yet, you should).
While admitting Brenna's point, however, I still maintain most typos in ebooks could be eradicated if authors proofed more carefully. Not all, mind you. But most.