Apple have a rule against making apps that "are simply websites bundled as apps". You should probably mention that somewhere on the website to avoid having a bunch of people pay for a developer account only to get their generated application rejected by Apple.
There's a difference between a full-featured multi-screen native-feeling app implemented in a webview with Phonegap and a wrapper around an existing web site. You can do a lot of interesting things in a webview and come, for some purposes, reasonably close to a native look and feel. The previous comment was right to warn about Apple's policies.
(I spent some time playing with Phonegap but it was two years ago and a simple app took 30 seconds to launch on an iPhone 3GS. Probably significantly better now, and faster as hardware has improved.)
Phonegap isn't about merely packaging a website as an application. It's about using front-end web technologies to build applications. There's no rule against that, even if it's possible to use it to break other rules.
What there is a rule against is simply packaging a website up as an application. That's what this service does.
While you may get away with it, there's a strong chance you won't. It's only fair to warn people before they spend money.
