======= Date Modified 16 Mar 2011 09:24:30 =======
I was faced with this dilemma as my PhD involved Smart phone subject matter and I didn't even have a mobile phone!
I have a PAYG HTC (bottom of the range) I put PDF's on it, take research related photos, the calendar is invaluable, there is a free dictaphone built in, it is sync'd with my laptop and I get all my email through it. I use the alarm clock to wake me up in the morning and routinely access Ebay and Tweetdeck with it. It cost me £150 and I have put £10 in credit on it since October last year, I use free WiFi alot.
I would look at the operating system closely, I chose Android because there are loads of free Apps for it, dictionaries etc... outside of these points they are, in my opinion worthless ;-)