virginmifi_thumb[1] I’ve written about using a MiFi or similar device—pocket routers that channel cellular Internet into secure WiFi—to “retrofit” 3G-style everywhere-connectivity to WiFi-only e-book and other devices. While there is a monthly charge for this Internet service, unlike the Kindle and Nook’s 3G, it is available for many more potential uses than the freebie connectivity baked into those devices. You can use it with any or every wireless device you own—at the same time.

At least, as long as your bandwidth holds out. At the time I wrote the aforementioned article, Verizon was offering 250 megabytes per month for $40, or 5 gigabytes per month for $60. A few other wireless providers have entered the fray, but most of them have similar caps. (Clearwire does not, but is currently limited to a relatively few metropolitan areas—nothing like the cellular network coverage of competitors.)

Even AT&T recently went from unlimited bandwidth for its iPhone and iPad plans to 200 MB for $20 or 2 GB for $25 bandwidth caps, and some people have been making noises about how bandwidth caps will probably be implemented for all mobile services sooner or later.

However, Virgin Mobile has decided to shake up the mobile bandwidth market, in a big way. Back in June, I reported on the prepaid contract-free MiFi broadband services Virgin was beginning to offer. At that point, bandwidth plans ranged from $10 for 10 days of 100 megabytes, through $60 for 5 gigabytes per month. Yesterday, however, Virgin announced it was going to keep the $10 plan but replace all its others with a single $40/month plan for unlimited bandwidth.

Think about that. That’s unlimited bandwidth for mobile data applications of any kind, not bound by contract lock-in so you are free to start or stop at any time—for less than the cost of many capped cell phone voice plus data plans.

With that plan, plus a $150 MiFi, WiFi Kindles become 3G Kindles*, iPod Touches become iPhones*, WiFi iPads become 3G iPads*, and anywhere you can get cell signal becomes a place you can surf the ‘net on your laptop to your heart’s content. Granted, it’s at 3G speed, but that’s still a lot faster than dialup. All for just $40 per month. (That loud “whoosh” you just heard was the sound of iPhone users’ collective intake of breath as they realize just how much they’re being gypped.)

(* Well, all right, almost. Poetic license, sheesh.)

This has the effect of making the iPod Touch 4, assuming it turns out to be all that we hope, an even better competitor to the traditional cell phone than ever. With VOIP apps and Facetime, plus a $40 per month data-only service, tech-savvy users can replace the cell phones in their lives altogether, or at least reduce reliance on them to cheap bare-bones pay-as-you-go devices on which to receive calls when their MiFi isn’t running.

Cheap, affordable mobile WiFi could bring some pretty major changes to the mobile device field—perhaps even to e-book readers. After all, if e-books can be downloaded anywhere, even on devices that don’t have 3G built in, those more expensive 3G devices might, in turn, lose some of their luster. Even if you don’t have to pay for their connection, if you’re going to be paying for a connection anyway for laptop use you might as well save the money up front.

And it’s possible that price pressure from Virgin might well drive prices down at its competitors. It will be interesting to see what happens.