Fashionable though it may be to bash AT&T’s 3G data network, its performance is, to an extent, a victim of its own success. Subscribers with the AT&T-only iPhone consume way more data than people whose phones cannot run a wide variety of apps, and as a result, AT&T now limits the amount of data iPhone owners can use each month.

The same approach is likely coming to competing cell networks and Android smartphones, now that two studies have found that Android users consume more data than iPhone users—and because Androids across various networks now outsell the iPhone.

First, Validas found that Verizon Android users consume 25 percent more data than iPhone users. Now, a Rhythm New Media study (via Giga Om) reveals that Android users are far more likely to stream video over 3G than iPhone users, who tend to watch more via WiFi.

The most obvious reason that Android users watch 72 percent of their mobile video over a 3G connection, while only 56 percent of iPhone video streaming happens via 3G, is that in addition to AT&T’s relatively new data limits, Apple and AT&T have taken steps to discourage users from streaming video over 3G. They do this by requiring some apps to use WiFi rather than 3G, like even Apple's FaceTime, while banning others completely. Another is that Android can be more readily customized by users to allow for greater tethering opportunities.

When Android is selling faster than iPhones, and Android users consume more bandwidth than their iPhone counterparts, other wireless networks have two huge incentives to begin metering Android wireless plans just like AT&T did with the iPhone, rather than continuing to allow Android users to download and stream unlimited data to their hearts' content. And such limits don't have to be reached in order to achieve their aim of reducing wireless bandwidth consumption; the fact that a limit exist is, in itself, enough to make users think twice before streaming a long video or using a data-intensive app.

This development marks another distinction between the wired (as in DSL or cable) Internet, where users can generally do whatever they want with their connections, and the wireless Internet, where strict rules are enforced, where access providers—first AT&T and likely soon the rest—openly limit the amount of data you can suck down in a month, and where net neutrality may not apply.

If you're contemplating the purchase of an Android phone, you should probably get a move on. Nothing is impossible, of course, and one of the Android carriers could somehow offer unlimited bandwidth forever. Nonetheless, the likelihood that Android users will continue being able to sign up for unlimited data contracts grows slimmer with each passing day.