Scrabble has been implemented on the iphone - and it works really well. You can play against the computer (and choose the level of your opponent) or against a real opponent using Pass 'n Play. If they have an iphone, you can also play peer-to-peer using wi-fi. To play, simply drag your chosen tiles onto the board, which automatically zooms in, pinch and drag to zoom in and out. Useful options include "Best Word" which will suggest the highest scoring word with the current rack. You can shake the iphone to shuffle the letters on your rack - or just tap the appropriate button.
Tips for playing Scrabble using the SOWPODS English and American dictionaries