Prepare to put in plenty of time...
I'm a .NET developer (both C# and VB.NET) that recently started building iPhone apps. I can tell you that very little you've learned from VB will transfer over to the iPhone/Mac world. The good news is that the most important thing - understanding the logic needed to program - is 100% transferrable. Other than that though, be ready to re-learn everything. Xcode takes a while to get used to - particularly if you're familiar with more advanced features of VS.NET. The Objective-C language itself looks odd at first, but after you write a sample program or two, it'll start making sense.
The two things that'll take a while are:
1) learning Cocoa Touch (the analog of the .NET framework...sort of)
2) learning memory management (biggie...no garbage collector like .NET)
The easiest thing to do to get started is to grab some of the samples from Apple and try to modify them. As you try to do modifications, you'll need to read through the Apple docs - you'll find out that Cocoa is quite powerful, but getting accustomed to the new way of doing things just takes time.
It really it's *that* hard to write an iPhone app, BUT make sure you go in expecting there to be a fairly large learning curve right at the start...once you get to a certain point, things are much, much easier...so just stick with it.
- Adam