US carriers have started blocking an app that allows Android smartphone users free wireless tethering for other Web-ready devices.

The application, “Wireless Tether for Root Users,” is still available on the Android Market. But if you have a phone that’s on the Verizon, AT&T or T-Mobile networks, you won’t be able to download or install it.

Try and access the app from the Android Market, and when prompted to choose which device on which to install it, you’ll only be able to select Sprint-carried devices or WiFi only products.

Wired.com attempted to install the application on a number of devices, and was only able to do so on a Sprint-carried Evo 4G and Galaxy Tab, as well as WiFi-only products. Our Verizon, T-Mobile and AT&T devices were unable to receive the app for installation.

A spokesperson from Verizon confirmed the application is not available to Verizon Wireless customers. AT&T, Sprint, T-Mobile and Google did not immediately respond to requests for comment.

If you’ve rooted your Android phone, using it as an Internet hotspot for your other devices is one of its biggest perks. When nary an Internet-enabled cafe is nearby, apps like Wireless Tether allow you to use your phone’s 4G, 3G or 2G data connection to access the Web from another connected (or “tethered”) device, like your laptop or WiFi-only tablet.

The best part: you bypass the monthly fee that most carriers charge for wireless tethering on devices (the standard monthly charge for wireless tethering across the big four US carriers is $30).

That’s why blocking the application makes sense in terms of a carrier’s bottom line. Missing out on that extra cash while its customers consume its precious bandwidth for free isn’t attractive to carriers.

This sort of interference isn’t anything new. After being fed up with jailbreaking iPhone owners who used similar apps to tether for free, AT&T decided to call their bluff. In March, the company started sending warning messages and then automatically billing customers who were suspected of using unofficial tethering apps. AT&T said it would not charge for the service only if a customer immediately stopped using his or her unofficial tethering app.

To the resourceful Android phone owner, these carrier actions won’t be a huge problem. Although the app won’t be available through the official Android Market, you can go directly to the developers’ site and sideload the program onto your device. And if you’ve rooted your device and have a mod like Cyanogen 7.0 installed, there’s already a wireless tethering option built into the program interface, eliminating the need to download another app.