Before expressing my opinion of Phonegap, I need to issue a little disclaimer saying that I am, by no means, a web developer. For me, writing JavaScript and CSS is like pulling teeth. That said, I don't like Phonegap.
There are a few very appealing aspects of the framework, mainly the "Write-Once-Run-Anywere" appeal and the "I-Don't-Have-To-Learn-Objective-C?" appeal. The framework can even give you access to some of the hardware features like the accelerometer. All of these are valid, to a certain extent.
That said, none of the great iOS apps are built using any kind of cross platform framework. There are so many subtle and elegant UI interactions that are handled for you by the native UIKit controls that many users and developers take for granted until they're missing, like the slew of incredibly detailed view animation curves and scrolling bounces that aren't there in the web-view based UI. You could toil to reproduce them perfectly, but for that time investment, why not just go native?
A second point to note is that the support community for Phonegap-built iOS applications is drastically smaller than natively developed applications. If you're one to rely on the community during projects, this should also give you pause.
In the end it comes down to the quality of experience you want to deliver. The upper bound is lower with Phonegap. If you're ready to accept a less-than-excellent user experience in exchange for reduced development times and increased portability, than it is a very viable framework.
As subtext for any who are interested, Marco Arment and Dan Benjamin recently discussed this topic. You can listen to it here.
