How hard is developing for iPhone?

It's rather difficult to answer your question owing to the fact that often this is highly subjective in my previous experience.

1) Generally the effort is much lower than the one required when using a different platform. Those acquainted to software engineering principles including the use of design patterns etc will find that the SDK is built around all of the common abstraction we are used (except a very small part still using C style procedural APIs).

2) The learning curve is steep for people rolling this on their own, is really easy for people being taught on the matter. A fast course style exposure to the SDK and tools (say 40 hours total) it's usually enough for people to become proficient enough.

3) There are no hardware issue to be taken into account, at least in my experience. As already pointed out by Zoul, provisioning the devices takes some time to get used to. The submission/review process is in my opinion a little easier.

4) Selling is as difficult as it is on other platforms. But if you have got a really brilliant idea, then you usually sell many copies of your software. Or, the idea may not be so brilliant, but the software you develop is fundamental for a specific field targeting people always on the move etc. Just developing something without a clear target is the perfect recipe for disaster.

Very subjective. A one person app developer can develop a medium sized app. How long will it take? Depends on how much free time the developer has and how much experience with Obj-C.

I learned some parts of the SDK in less than a day. I still don't know the entire SDK, as I haven't needed to. I doubt that any one programmer would want to spend the time to learn the entire SDK. For example, if you aren't doing anything with the accelerometer, why study it?

You need to roll up your sleeves and delve into it yourself to see how long it takes you. If you are asking for your team, then you will have to judge how well their expertise will apply.

As for selling on the iPhone, there are some easy aspects, like not having to worry about packaging or salespeople, but you still have to sink money into marketing or no one will find your app in the almost 95,000 apps on the app store today.

If you're asking because you keep reading that it's an easy "get rich scheme", then I'd say you're in for a surprise. Despite the reduced overhead in some areas, and low start-up capital, it's as much work as any other software venture, since the ratio of team members to work to be done stays about the same (the economics of a $2.99 or $4.99 or $9.99 apps forces you to have a smaller team).