iPhone data caps got you down? Onavo offers a solution by compressing all your data so you don’t use as much of it.

The free iPhone app claims to save up to 80 percent on a monthly data allotment. Just run the app in the background, and Onavo gets to work on crunching those bits.

It’s a useless app for the Verizon iPhone, given that data is unlimited for the time being, but on AT&T, Onavo is a potential lifesaver. An 80 percent savings would effectively give you 1 GB to play with on AT&T’s 200 MB, $15 per month plan. The carrier’s 2 GB, $25 per month plan would expand to 10 GB with Onavo behind the scenes.

Of course, real-world savings aren’t as much as the maximum that Onavo advertises. I haven’t tested the app extensively myself, but TechCrunch’s Roi Carthy reported 75 percent savings on Maps, 64 percent on web browsing and 12 percent on e-mail. Still, that’s not bad, and Carthy says there wasn’t any noticeable slowdown in data speeds. Just one caveat: Onavo doesn’t work when the iPhone is tethering or acting as a personal Wi-Fi hotspot.

All of which begs the question: Why aren’t wireless carriers employing this technology themselves? Carriers already use compression to optimize traffic, so I’m curious what’s different about Onavo’s technology, and why the carriers aren’t using it yet. (I’ve pinged Onavo with these questions, and will update if I get an answer.) We could be looking at acquisition bait. [Update: See comments from Onavo below]

Otherwise, Onavo doesn’t have a clearly-stated business model yet. The app is free, but it may move to a subscription model in the future, which for users would create a conflict of spending money to make money. Onavo’s privacy policy also suggests that it may transmit aggregate user data to partners and affiliates, but it won’t include any personally identifiable information or sell user data without explicit consent. If that doesn’t bother you, a world of data frugality awaits.

Update: I spoke with Onavo spokesman Dvir Reznik, who explained the service in a bit more detail.

In addition to across-the-board compression for things like images and e-mail, Onavo works with individual apps to throw out unnecessary data. For instance, if you’re using a Twitter client, Onavo could use cached profile pictures instead of downloading them every time you refresh your news feed. The company targets the most popular iPhone apps for compression, since it obviously can’t optimize every app in the store.

As for business models, Onavo plans to charge a subscription fee for the data-saving application. The part of the app that monitors data use and tells you how much you’ve consumed will remain free. Onavo has also spoken to wireless carriers in the United States and elsewhere, but more on the topic of monitoring data use and giving users the tools to cut back on bandwidth.