I have been very frustrated with the xperia play which i just bought couple days ago. The problem I have right now is that apparently the battery seems flat, but phone stays off / wont turn on with all the try advised from the sony support gude (see below)
If the phone doesn't power on:
Charge your phone's battery using the power adapter or a computer. It may take a few minutes before the battery icon appears on the screen when you connect the phone to a power source. Press and hold down the Power key to turn on the phone. (I did let the phone charge with the power adapter for more than 2-3 hrs and see no icon. Also, the power key indicator stay flashing while apparently charging.)
Remove the battery cover and make sure it is a Sony Ericsson branded battery. If not, this may be the reason why the phone will not power on. (100% sure is original sony battery out of the box)
Remove the memory card and restart your phone. If your phone starts without an inserted memory card, the memory card might be defective. (Did try, but still the same.)
Make sure that the gold prongs on the battery and the phone are not bent or broken. If the battery is damaged, replace it immediately. (bettery never get dropped or bent since I put it in at the first use.
At the moment, I am very close to the desperation as I do not know what may have caused the problem.