Advice on being a lead developer is welcome. Advice on being a lead developer of an Objective-C team more so. Unfortunately, I don't have experience in Objective-C (I suppose the first thing to do is to learn it). Also, I've never been a lead developer before. I do have about 6 years experience in developing applications in Java, Flex and C#.
The best thing you can do as a leader is to stay out of other people's way.
Your job as lead is to maintain an overall "big picture" view of the project, while being able to drop down to any level. The best leads I've seen are always available to help, always try to be involved, but don't step on their team member's toes. Some ways to achieve this:
I wouldn't worry too much about the choice of languages. You can learn any language you need to; experience on other projects (even in other languages) will only help you. Just remember that's it's more likely that your team knows the language better than you, and be prepared to defer if necessary. You will have to work hard to bring your own skills up to par.