Where can Apple go from here?

For anyone remotely interested in consumer technology trends, the single greatest research tool on planet Earth is the New York City subway system.

New York City is a world center for art, fashion, business and media, and the subway is jam-packed full of forward-looking people who tend to get in on new trends early.

So what happens here leads what goes on elsewhere.

Now, I've been commuting daily on the subway for more than 20 years, which means I've seen it all. And I'm not just talking about the fights, the drug use and the nudity, I'm talking trends.

I saw Apple iPods all over the subways well before they became mainstream products. Same with the iPhone, the Amazon Kindle, and "Guitar Hero."

I also saw several brands and product categories peak. There were times when it seemed like on the subway, every adult had a BlackBerry and every kid had a Nintendo DS. Looking back now, it seems that that's when they peaked. When everyone has something, there's just not much room to grow.

The reason I bring this up is because of growing worries over Apple having peaked, highlighted by a Citigroup report saying that iPad and iPhone demand is slowing.

Now, as an Apple shareholder, I don't want Apple to have seen its best days, but it's a scenario I have to consider based upon what I see on the subways.

Ten years ago, it wasn't common to see Apple products in public. Even iPods were rare back then. But today, it seems like everyone on the subway in New York has an iPhone or iPad.

Kind of reminds me of when everyone had a BlackBerry ...

Michael Comeau is a columnist and buzz editor at Minyanville.com.