Since you know C++, then it won't be that big of a switch for you to learn Objective-C. While VB to Objective-C is a much bigger learning curve of course (but I think VB to any C syntax-type languages, which are most common, is hard for most).
You'll probably already have a handle on pointers, memory management and the like so Objective-C, while it does things in it's own way, it won't be alien to you. Objective-C is layered on top of C, so if your knowledge of regular C is good then you've got a head start. You just need to get familiar with the Objective-C syntax, which differentiates itself from C, like using [] for calling methods, memory management (through the use of retain/release), etc. The apple dev site has a primer on Objective-C. Read browse through a Objective-C book, like the one by Stephen Kochan.
For people coming from scripting languages, ie PHP, VB, etc, they have more challenges to overcome to learn Objective-C but C/C++ developer who deals with lower-level coding regularly, they should have the easiest transition, IMO.