The Dell Axim X30 and the iPAQs are very popular and good quality units.

A Bluetooth phone would certainly do the trick. You could use a Pocket PC phone too- it depends on whether you prefer convergence devices or dedicated ones. Do you mind talking to a Pocket PC rather than a standard mobile phone for instance.

So far, there have been no real PDA virii-- only one proof on concept on which never circulated much and required that you tap an OK button to install the virus! Not much of a threat.