If it's a "trend", it's too late
I disagree. In fact, if something is a trend, it's a guarantee that a market (of some kind) exists and it can save you a lot of time and research. Most big markets have large numbers of high value competitors and they're not all pioneers. Indeed, most companies with significant turnover were not pioneers in their field - they came in with "me too" products that merely improved upon the existing popular ones before they became too entrenched.
The trick is to get in when something becomes a trend but before the masses realize it's a trend and you have hundreds of competitors making similar improvements to the core products and services as quickly as you are. As well as being my belief, it's one of the core tenets of Michael Masterson's tackily-named (but a great read nonetheless) "Ready, Fire, Aim: Zero to $100 Million in No Time Flat."
With your reference to the App Store, I suspect you might agree with this, but have a different idea of what a "trend" is. I consider the App Store and developing iPhone apps to have been a trend in late 2008, not long after the earliest developers were starting to share revenue/sales figures, but before the masses had figured out it was going to be a giant cashcow. Now developing iPhone apps is just "what you do" rather than a trend, IMHO.