Why buy an iPhone when you already have a Treo? This is an excellent question, but like a lot of things, it repeats history in some interesting ways. As time marches on, gadgets keep changing, and sometimes, improving (not necessarily both!). I've lived through many of these "revolutions" and many of the busts.
I like my Treo a lot. I lusted for the 600, which I codenamed the "360" as soon as I heard Jeff Hawkins describe the earliest Treos 90, 180, and 270. As a v2.2 guy, I figured the next version would be called the 360 (a tip of the hat to the iconic IBM mainframe computer of "the '60s", which lasted well into the '70s). As soon as I was out of contract with my clunky Verizon handset, I got a Treo 600 a set aside my Palm Vx (in fact I had TWO for awhile, as I compared the Verizon and Sprint data services). One gadget to replace both a phone and a PDA -- Nirvana! And, as soon as the Treo 700p was available, and my 2 yr contract had expired, and I could get the appropriate discount, I satisfied my desires and upgraded.
But the Treo requires a commitment to function over form, and there are some limitations. Maybe the design geniuses at Apple could do it better, and it would be truly a new device -- with the multi-touch screen, and a different style of interface -- geeky, but cool, and easy to use as well. I've been unsuccessful in convincing my wife to use any kind of PDA. Maybe the iPhone would work for her (or both of us?).
So, I had to try it. And I was willing to invest $50 or so to try it out, even if I took it back. And the hype and the line-waiting, etc, might be fun. I've heard it all compared to Woodstock. And, Cali, [assuming political put-down voice] I was AT Woodstock (well, actually "Goose Lake", a Michigan knock-off festival in '70), and having stood in line and tried out the iPhone, it ain't nearly as "far out" as that was ... but it is "way cool"!