The devices we use to access the Anywhere Network are truly legion — and getting more so. Gartner Group got a lot of headlines this week noting that one billion PCs are now in use worldwide. As one of the articles notes, that means that there’s a PC nowadays for one out of every 6.7 people on Earth. That’s an amazing accomplishment.

What’s I’ve been struck by lately, though, is how last century this business of counting PCs is. Yes, they are incredibly important devices. But PCs took an amazingly long time to reach this billion units from their humble beginnings with the Apple I introduced in 1976. Meanwhile, another personal device technology — call them PDs — invented around the same time has grown to more than 3.44 billion units, according to Yankee Group’s latest Global Mobile Forecast. That’s 1 PD for every 2 people on Earth. Of course, we know PDs better as mobile phones.

Today’s mobile phones make calls, keep our contact lists, take pictures, exchange email, do instant messaging, and can even help you navigate — and yet unlike PCs, these devices fit in our pockets. Unlike PCs , we rely on our PDs to always work, to be secure, and to become electronic extensions of ourselves. For example, in Japan, many teenage girls decorate their cell phones and their nails to match — you don’t see that happening with PCs. And, according to a recent study, people are more likely to leave their homes without their keys and cash than they are without their mobile phones; PCs, not so much.

And Yankee Group’s research shows that this trend of PDs outgunning PCs for the hearts and minds of consumers isn’t likely to stop soon. As we’ve noted in our Yankee Group Teens and Technology surveys, texting has wedded an entire generation of teen-age users to their mobile phones with a loyalty that their PC-centric and voice-addicted elders just don’t understand. And an upcoming Anywhere Enterprise report will discuss the future of Anywhere mobile applications. Hint: it’s easier to wean executives from their laptops than their BlackBerry PDs nowadays.

So bravo to the billion PCs in the world. Just don’t expect the 3.5 billion PDs on the Anywhere Network to wait for them to catch up.