Battery "Usage" time disappearing????

I've been monitoring my iphone 4 battery usage times religiously since I got the phone June 23rd. Until the last week I have been averaging roughly 5 hours and 45 minutes of "usage" and about 15-18 hours of "standby"(Great battery life in my opinion). Basically charging my phone in the late afternoon on the second day after a full charge.

The last several days I have been checking my usage throughout the day because I have noticed my battery draining faster than usual. The last two times I have looked I have shown somewhere in the 2 hour 30 min range down to 20% battery. When I check again the "usage" and "standby" times have magically disappeared. The phone shows blanks and "usage times will be shown after the iphone is fully charged." I charge my phone fully 99.9% of the time so the usage times should almost always show up.

Why are my usage times disappearing mid cycle?

Iphone 4, ios 4.0. (Purposely have not upgraded to 4.0.1.)

 

I've rebooted the phone a few times. I haven't tried doing a full restore through itunes yet though.