There are always going to be new models of phones/computers/pda's on the horizon but one of the major reasons that I jumped on the iPhone when I saw the price drop was in my little old opinion Apple does a nice job of updates for all their devices.
Take a look at what happened when the move from WinXP to Vista came about, does everyone remember all the crazy new hardware requirements that were involved? Now I am not going to bash windows. I just want to make a point and I will get to it in good time. I am a windows user have been ever since Win 95 crashed onto our little world and changed everything, but I did have a Powerbook for a while, and though there is a long story over why I no longer have it, one of the things I loved about it was even when new flavors of OS X came out, I didn't need to upgrade every component in my system nor did I have to rush out and buy a whole new machine to get it to run. I was running a 12 inch Powerbook with an 867 mhz and 512 megs of ram and a 40 gig hard drive, and you know what I put every flavor of OSX that came out on it, and they all ran smooth as glass.
Now on the windows side of things, every single time that a new OS comes out I need to purchase a whole new rig, whether it be a notebook or a desktop, cause some new fancy feature of windows requires more ram, or a faster special processor or a better video card, and the list goes on. I noted the same for flavors of WinMobile. Certain windows mobile devices are stuck with their particular flavor of WinMobile, either WM5 or WM2003SE and so on and so forth, because for one reason or another there isn't enough memory or the processor isn't fast enough. The wireless component isn't supported and a laundry list of issues. Granted this isn't going to always be the case, some device can make a step up like the Treo 750 from WM5 to WM6 if AT&T and Palm and get it right for us U.S. customers.
The iPhone, its just like any mac I have ever owned. Apple releases and update, and you get a notification when you go to sync that there is a new firmware, flash it, and you got that new feature. Now I am not saying that via a firmware update you will get 3G or more storage, but the software side, in this Apple is very smart, they can continue to be inovative and change the face and feel and sometimes even correct mistakes.
So do I think there will be new featues, always, without a doubt, but current iPhone users I don't think will be left in the cold. This is probably the first mobile device that I've owned in 8 years that I haven't felt it was obsolete when I walked out the door.