Many, many people I know are doing iOS development as a sideline. Some for a pure break from the day-to-day routine of raising a family, some for fun, some even consult for side income.
Most of the people in the large group above want to learn mobile development and pursue it because it is invariably more interesting than whatever technology (e.g. Java/C#) they implement in their day jobs. It's ok to have it as a sometimes-on, mostly-off hobby.
A very important point, I believe most of the jobs in iOS are at startup-type companies or startup-type-minus-equity skunkworks in big companies (e.g. get that project done in 4 months) - no 9-5 8 hr days - its all about get 1.0 out ASAP. Realize that having a 9 to 5 relatively stable job is a big benefit - especially if you've grown used to it. And it gives you the money/security to pursue something more interesting, if you so desire (for trading time - lots of it 40 hrs + commute...) A tradeoff that many, many people make.
I know a minority of people who do iOS development full-time. And most of them don't worry about cash-flow issues/healthcare because they have understanding spouses who have relatively stable careers and good insurance.
And those who do it - make the majority of their income from consulting (by making apps for companies who want an app - its an ego thing - if X company in Y niche has an app, they all want an app - Public Libraries for example).
And some of those who do mostly consulting have done enough hours that they are starting to realize they want to build product. There are a lot of niches out there - it's all marketing and connections and luck. Work for money vs Work for personal ROI.
I have met very few who make a lot of money (relative to their income they live on) from iOS or Android apps. But they do exist. And they are surprisingly humble (e.g. if someone is boasting about their app's sales - they are doing it for a reason - for PR, for EGO - not purely for selfless sharing)
If I were you, I would suggest diversifying your mobile development skills to include Android experience. Android is going to win in the larger consumer market and having the skills to build an Android app along an iOS app can't help but make you more marketable and billable. Truth be told, doing both Android and iOS development is possible - if you tag-team projects with as long a duration as possible. Your head might explode going from Android to iOS in 8-hrs (I know you definitely get a headache from personal experience)
Also, the more you can grasp about art/design - make things look good - the better you positioned you are to execute on a mobile idea.
Study the apps in New & Featured. What makes them tick? Read the comments. Can you make something like that? If not why - lack of design/domain knowledge/time/resources? It doesn't have to be perfect - and most people with iOS apps use them as a portfolio piece - not as their financial portfolio.
It is possible though to win the Appstore lottery (New & Featured) - but to get a ticket - you have to have something that is almost at the level of polish that it would be camera-ready for an Apple TV commercial.
And the more time you invest into mobile development, the more secure you should feel about job loss due to layoffs - it should give you more bravado the more consulting hours and successful mobile projects you manage to do. But diversify, diversify - don't just code all day - go to an art gallery, enjoy life.
I'm already working on a kind of 9-5 doing only iOS. I'm in Argentina that right now have a lot of work inflow because our low cost and less culturally differences than asian countries and almost in the same time zone.
I failed in a couple of startups of my own and though there's virtually no support to start-ups here, I'm no sure I have an eye for marketable things. I don't really care for the AppStore lottery anymore.
So as I'm realizing I'm less an entrepreneur than I thought I was, it may be a good thing to start steering to safer waters. After two years of J2ME and one of iOS, last week I had to think twice to make a query with a group by, and got kind of scared.
I'm not worried about being able to keep up with technology changes, but I don't want to get specialized in a market that has no depth.
If you have a great idea (i.e. mobile application) in mind, then by all means, dive in. Without a doubt, users can still benefit from innovation.
Evidence tells us that the majority of successful new startups are produced by developers in the 20-35 age range. Some point to the open-mindedness of younger generations as a reason why. I don't think the "can't teach an old dog a new trick" is always applicable or necessarily true, but then again, it wasn't synthesized from theory.
Being a mobile developer is certainly a choice with a future, but you have to decide if a) those are the types of companies/consulting work/independent projects that you can make a career out of and b) how ready you are for the device/platform rapid iteration that should continue for the foreseeable future.
