Also, to get to the last read message in a thread, you have to tap directly on the box on the right with a red dot. Anywhere else, and I believe it takes you to the beginning of the thread.

Additionally, it's by page vs. post. At least for me. Reading one or more posts on a page will register the entire page as read. That only applies to whole pages, though. If a page was partially filled and you read to the end of that, the next trip back takes you to the correct spot.