AnySoftKeyboard - Pali & Sanskrit Language Pack
Standard QWERTY keyboard layout with Pali and Sanskrit diacritics and Pali dictionary (about 75,000 of the most common word-forms in the Pali texts).
Install AnySoftKeyboard first, and then select the desired layout from AnySoftKeyboard's Settings->Keyboards menu.
Note, you will need a special font to actually display the diacritics properly. Instructions on one way to do this are here (requires root):