Virgin Mobile has upped the ante on cellular data: Despite being owned by Sprint Nextel, Virgin Mobile is challenging all four major US carriers with an as-you-need-it, no-contract $40 unlimited 3G data plan. The plan lasts for 30 days. Virgin previously had four levels of service topping out at 5 GB for $60 used within 30 days. The new tiers are $10 for 100 MB over 10 days or $40 for unlimited data during a 30-day period.

Because Virgin Mobile also offers the MiFi cellular router for a low price ($150, no commitment), it now has a killer offering. Use a MiFi with an unlimited plan and avoid the overage fees or throttling from every other competitor.

This also guts tethering plans. I'm an AT&T customer with an iPhone 4, and I also own a 3G iPad (with no current active service plan). I typically now travel with the iPad and activate a plan on the road. I had figured on my next trip in which I needed a laptop, I would switch to tethering on my iPhone 4 (from a $15/200 MB plan to a required $25/2 GB plan plus $20 for tethering). That now seems unappealing.

Instead, I should pay the $150 for the Virgin Mobile MiFi, and pay $40 whenever I'm traveling. Then my iPhone and laptop can both use Wi-Fi to access Sprint's 3G network, and if I'm traveling with colleagues, I can share access with them as well.

Sprint recently dropped its MiFi offering (so far as I can tell) in favor of the Overdrive 3G/4G, which works on its Clearwire division's 4G WiMax network (no limits on use) and the 3G CDMA network with a 5 GB cap. (It's $350 upfront or $100 with a two-year contract at $60/mo.) You can also go to Clearwire and buy a similar product (the Spot 4G+) with a $55/mo service plan for the same terms.