I ported the rather nifty Psion 5 app called PhoneMan to the Palm pilots. It allowed managing address books and sending and reading sms messages. I never quite got around to finishing the picture message functionality.
I even had sync working with the Bluetooth module and an Ericsson phone.
I had a couple of Palm devices and then a Handspring Visor, but it was HTC Typhoon which first felt like magic to me. Phone, web, email, MP3s, podcasts, RSS feeds all in a tiny device with what was a pretty big screen for a phone in those days. I had a pretty punishing commute, 2+ hours each way by train, but it became my favourite time of day.