OK, the OP doesn't tell me how to get into the bootloader and because this isn't my phone I had to look it up. IOW, the steps aren't explanatory enough.
Anyway, this should be a stock locked T-Mobile phone with Gingerbread. When I hold Vol - and Pwr from a powered off state it takes me to the bootloader menu. When the PG58IMG.ZIP file is on the root of the SD card, I see that it automatically says something quickly and disappears, leaving me back at the normal bootloader menu as if there were not even an SD card at all. By repeatedly picking FASTBOOT then BOOTLOADER to trigger the flash of info repeatedly, I managed to piece together that it is showing something like this:
SD Checking...
Loading...PG58DIAG.zip
No image
Loading...PG58DIAG.nbh
No image or wrong image!
Loading...
PG58IMG.zip
[New Screen]
Checking PG58IMG.zip
This is a stock Gingerbread ROM that hasn't been booted since sometime this summer when the owner shattered the glass and damaged the LCD, in case that helps determine how up-to-date the software might be. I just repaired those other issues and and noticed the terrible touchscreen responsiveness and hoped to fix that too, but I can't use the T-Mobile update considering that I have no SIM card (in owner's other phone for the last several months).
I'd like to have this taken care of before I return his phone. Any help is appreciated. Thanks!
Edit: Somehow got the OTA update over WiFi and the screen works great now. Thanks guys!