Sebastian Goscik has created an app to allow Sony Ericsson Xperia PLAY users to take photos or shoot video by utilising the R button as a physical shutter button. The app called Camera Mod for Xperia PLAY can be found on the Android Market.
Goscik modified Google’s stock camera app to allow the R button to be used, so everything else that you find on your current Xperia PLAY camera remains the same (apart from some minor positioning issues and increased font size). If you find it useful, then there’s also a donation version in the Android Market.