When searching in personal messages, asides from the user, I am requested to search for a particular word or phrase. It would be great if I had the option to leave that field empty. For example, I may not be looking for any particular conversation I've had with user X, but I want to get all the messages we exchanged.

Another feedback: When searching for PM, I want to be able to search to my Outbox only, instead of my Inbox.