I've got a 2G iPhone and here's what's going on with my mobile mail app:
1) On Friday the 13th, I'd noticed that my iPhone lost it's charge in only a few hours... I didn't think anything to wierd about it.
2) I recharged my iPhone, and as soon as it finished charging, I noticed that my Mail app was showing that I had about 3000 unread messages... very strange since I normally have about 20-40 unread messages.
3) I have 3 e-mail accounts on my device, one a work account(exchange), and two gmail accounts. One of those gmail accounts was showing something like 2900 unread messages... very odd.
4) I opened the in box for that account, and poof, app crashed.
5) I rebooted the device (hard reset), and was able to open the in box for that bad account. In there I saw a message with the no subject no sender symptom.
6) Now comes the wierd part... all the messages below that bad message were duplicates of each other... so let's say the next message was "Hi" from buddy@mail.com, that message was repeated hundreds of times, and so forth.
7) At the same time I noticed that the app was continually trying to get more mail from the server, and what it appeared to have been doing is getting the same messages and putting them in the in box list.
8) I tried deleting the account, re-adding it, but the duplicate messages, and the no subject/no sender message would keep coming back. I assume thats why my battery kept discharging, since it's been continually trying to get the messages.
I haven't done a restore yet, because I wanted to save the failure information for the de-bug folks at Apple.