I made the same switch this year. I've been a web developer since 1998, and last April got hired by a company that basically did the same thing--sold some iPhone projects and then figured out how to deliver them.
It was a solid month before I wrote ANYTHING that ended up in production code. That month was one of the most intense periods of learning I've ever had in my life. I went home each night with my brain leaking out of my ears. I was literally dreaming about iPhone development--I remember a dream one night in which a girl was wearing a one-piece bathing suit made out of a UITableView. It was scrollable and everything.
It's similar to web development in a couple aspects, and a couple aspects ONLY. Control flow within a method is similar. How methods get called is entirely different, but an if statement is still an if statement. It still has all the data types you're used to dealing with (arrays, associative arrays, strings, etc) but it calls them different things, and most of the time they're objects but some gotcha times they're not. It's MVC, but not the way you're used to thinking of. Doing web requests from it sort of feels like writing AJAX calls, though of course the syntax and control flow are pretty different (actually, the new blocks interface in ASIHTTP makes it closer to jQuery than it used to be!).
There are a couple projects mentioned in this thread (PhoneGap, Appcellerator) that basically wrap a UIWebView with your HTML/JS content and make a native app out of that. Compared to the native API, they're not much, but they do flatten the learning curve significantly, and for the apps you describe they might well be sufficient.