Boing Boing pointed out a great article by Cory Doctorow in Popular Science that makes a strong case for buying a Smartphone. He says that Smartphones like the Sony Ericsson P900, the Nokia 6620, and the palmOne Treo 650 are worth buying because owners can install their own software on them. This means the owner can use the web browser of his choice, the email client of his choice, and productivity add-ons that carrier-controlled devices are not likely to support.
This is an article that I wish I had written. He's absolutely right. If you care enough about gadgets to read Operation Gadget, you're probably among the select few who signed up for wireless data services on your mobile phone before there were any useful applications for it. You'd be willing to pay a few bucks more per month to enhance your productivity. You realized that linking your contact manager, calendar, and mobile phone together would improve your personal effectiveness.
I don't think it's our job to evangelize Smartphones to people who are only interested in making high quality voice calls, but many of the mid-range mobile phones out there are being artificially limited by some carriers. For instance, I'm not sure that any but the most sophisticated potential customers are aware of the limitations imposed by Verizon on their customers who purchased the Motorola V710. There's also a lot of commentary on the Internet indicating that mobile phones that ship with the Brew application platform rather than Java are more limited than they otherwise would be, from a user perspective.
You can avoid all of this by buying a Smartphone with a full-featured operating system. Handsets based on the PalmOS, Symbian, Windows Mobile, and Linux platforms are all good choices because they can be extended with third party applications that you install yourself. In this respect I think platform choice is critical, and you get what you pay for.