I've been doing PHP freelance development for the last 4-5 years and I'm starting to max out my hourly rate. So in 2010 I decided to transition to a new language. I played with Python and Ruby, but ended up settling on iOS, for three reasons:

• I'm enjoying the challenge of working on a completely different type of development, instead of another flavor of web development
• The demand seems higher right now than for Ruby or Python
• I see iOS developers charging $150 - 250 / hr

Whether these reasons are right or wrong, I've been learning iOS for the last year and I'm starting to get more work in that field. I feel confident that in six months (barring any major shifts in the ecosystem), I can be billing iOS work at $150 / hr or more.

However, I'm feeling that I should have done this earlier, that I've missed the boat, and that iOS development is going to dry up or get much more commoditized. Whether this is true or not isn't really my question (though feel free to comment). What I want to know is: what should I start learning right now so that I can be ahead of the curve in a couple years when the demand is far outstripping supply? What technologies or skillsets are going to be so heavily in demand in 2-3 years that you'll be able to charge $250 / hr or more and stay busy?

These don't have to be new technologies either...the answer could be iOS or COBOL or whatever.