If this hasn't been suggested before, I'll be amazed, but I'd really like to see this feature implemented. There are times when I'm on my phone and writing isn't practical, or even just not in a mood to respond right away, but still want to READ incoming mail. And there have been times that I've read a PM, and then forgot that I never replied to it. Mark Unread in PMs would also help for the people who prefer to play via PMs.
If this is in any way doable, please. It would be a huge boon.