I asked the same question in late 2009 ....
My actual experience was very different from when everyone told me.
Instead of just one app, I developed 10 (6 iPad, 4 iPhone) and four years later, they are still in iTunes and still selling although, now the sales are very small but steady (I track sales and a ton of other data with Prismo).
So far, I had close to 60000 downloads (all 10 apps combined) and I expect that the new series of Little Composers educational apps will do much better because back then, I just got started.
If you are serious about what you do, establish a name. A brand (so to speak) and develop under that name.
If (another "if") you can afford it, copyright each app and trademark your brand! Copyrights are cheap (I have six and each was $50 Canadian). Trademarks are a different story (I have one pending and am about to file two more). You can get one for a little under $1000 (depending on how many classes you file for and assuming you are in the USA).
Speaking of the good old USA.
82% of all my sales came and are still coming from the US followed by the EU, Great Britain and Australia. The rest is all over the globe.
iTunes can be a good way to supplement your income but don't expect to hit it big with your first app.
Those days, are over.
I think that hitting ALL the app stores and catering to all platforms should make it worth your time.