http://developer.android.com/guide/topics/ui/accessibility/apps.html:

Handling directional controller clicks

On most devices, clicking a view using a directional controller sends a KeyEvent with KEYCODE_DPAD_CENTER to the view currently in focus. All standard Android views already handle KEYCODE_DPAD_CENTER appropriately. When building a custom View control, make sure this event has the same effect as touching the view on the touchscreen.

Your custom control should also treat the KEYCODE_ENTER event the same as KEYCODE_DPAD_CENTER. This approach makes interaction from a full keyboard much easier for users.