Yeah... there's no way that the Seen feature is accurate. How does it know if you actually viewed the message or simply have Facebook running in the background and the message popped up without you even looking at it?
But a "read receipt" would be nice (although a lot of people hate them, lol)...
for that the Messages and the Chat would need to be separated again. I never liked it that they merged those 2 features... it really doesn't make sense.