I've only worked with Palm and Windows Mobile phones, but I've found it quite simple to use the Bluetooth serial port profile in the smartphone side. For the PIC I use a model with a USART (18F1330 mostly) and a BT-to-serial module. I looked at WiFi but it seemed too power hungry. That was several years ago so the technology might have changed/improved.

I'll be curious to know what you finally decide to go with.

Best Regards,
Paul