Battery life
For me, one of the most critical issues is battery life. I don't want to be constantly thinking about how long the power will last when there's a job to be done. There are two basic solutions to this problem: get more power, or use less of it.
Ensure that your new device has replaceable, rechargeable batteries. Often the available replacement batteries will have higher capacities than the standard ones supplied. Also, make a point of using mains power when it is available, or even 12V if you spend a lot of your time in the car. Generally you should be able to get adaptors to connect your phone into a car's electrics.
Certain functions are far more energy-consuming than others. Wireless communications are worth minimising, whether it be a wireless or a mobile network. Extensive Net surfing will seriously compromise battery life. In dark environments, a backlit screen is great, but again power hungry. Beware also of unnecessarily fast processors: extra features require extra energy.
For a PDA or phone, the OS along with basic applications will be stored in ROM. RAM is for your extras. If you are only looking for a communications device and diary then you will not be too concerned about memory; even 8MB of RAM will be plenty. More would be advisable if you wish to store video or MP3 files. Additional applications that might be required will, naturally, further increase RAM requirements. Just remember not to use your entire RAM for data storage. Like your desktop PC, a certain amount of RAM is required for working space. You will pay in terms of application speed if insufficient memory is available. Actually some devices will not allow the user to get at all the RAM. Remember that you are still in contact with the Internet. Files can be stored on a server and downloaded as required. Not only that, some PDA/phones have memory expansion slots for further storage by using memory cards such as SD or Memory Stick and other devices.
Processor
Remember that a faster processor means faster battery drainage. Decide how much of a priority you place on computational speed. Can you afford to wait an extra second or so to process image files? Do you plan to do complex spreadsheet, document, or image manipulations? Typically PDAs will be in then 120-400MHz range. By contrast the Portégé notebook and Stylistic tablet we looked at have 1GHz processors.
Security
The greatest security risk with PDA Phones is loss. Requiring passwords on power up offers some protection, but chances are you will leave the device on -- after all, it's a phone as well. Also, such features will not protect data kept on removable media which might be present. Consider encrypting valuable data. Better still, avoid storing valuable data on your PDA at all. Can you afford to have your address book in the hands of your competitors? How much data you store on a mobile device is likely to vary with the capabilities of the machine. Detailed documents may not be present on a device without capacity to edit them, unless they are imported for inspection.
Malicious code is not a serious risk as yet, but manufacturers of hardware and security software are taking the possibilities seriously. A Trojan called Backdoor Brador A has been discovered that affects ARM processors, which are used by a wide variety of devices, including those from Handspring, Motorola, Palm, and Texas Instruments. The Trojan can take complete control of a PDA and allow files to be stolen.
A virus, known as WinCE Duts A, has also been discovered (actually it was sent to antivirus software manufactures to illustrate a security flaw in Windows CE). It has no payload and won't spread without the user's express permission, but it proves the point that viruses on portable devices are at least theoretically possible.
There is definitely potential for viruses to become as serious a threat to PDA security as they are to PCs today. The threat is likely to be realised as PDA use becomes more widespread.
One of the main problems I see with all the phone/pda's is that they seem to forget about phone functionality and concentrate on the PDA side of things. For example handsfree dialling/answering. Most of the end user I talk to want the phone features first and the PDA features second.