In what can only be described as a real sphincter-clencher for Sprint customers, the nation's last true unlimited smartphone data provider has made a move that may signal the eventual end of that philosophy.

Yesterday, Sprint announced that customers on its Virgin-branded pre-paid arm, Virgin Mobile, will now be subjected to data throttling after 2.5GB of usage in a month. Sprint claims this will only affect 3% of all Virgin mobile data subscribers. The throttling will limit data hogs to a paltry 256Kbits/s once they've capped out at the 2.5GB mark in a single month - which is actually a little better than what T-Mobile does (dropping people to EDGE speeds).

This news will likely evoke a response from Sprint at some point - whether on their upcoming earnings call, or maybe even sooner than that. Unlimited is Sprint's marketing lifeblood, so we wouldn't expect them to begin throttling on their primary network any time soon. In fact, I'd expect a vehement denial of any move away from unlimited data by Sprint. But throttling and "unlimited" aren't exactly mutually exclusive.

We know how this song and dance goes - T-Mobile calls its data plans "truly unlimited," when it's clearly just marketing puff - consumers are throttled after reaching 2GB of data usage in a month. It's like having unlimited milkshakes - but after the first two, you have to drink them with chopsticks until next month rolls around. No doubt, this kind of advertising is highly tempting for a carrier looking to save a few bucks.