It was too good to last: Virgin Mobile's remarkable $40 MiFi plan with unlimited service will no longer be so remarkable. That $40 bought you unlimited data on Sprint's core (non-roaming) 3G network. Service lasted 30 days, an neither a contract nor cancellation fees were involved. The revised terms, for new plans activated starting 15 February, will throttle your usage after you pass 5 GB within the 30-day period. (The MiFi is a portable cellular router that shares a mobile broadband connection with up to five devices via Wi-Fi. The plan requires separate purchase of a MiFi from Virgin Mobile for $150.)
Just as T-Mobile implemented with their 5 GB, no overage charge plan several months ago, Virgin Mobile will restrict throughput to a low level (probably 50 to 100 Kbps, based on other carriers' actions worldwide) for the remainder of the period. You can immediately purchase another $40 plan, however, to reset the clock.
I assume Virgin Mobile came to the same conclusion that other carriers did. It's likely that up to the 97th percentile of users consumes under 5 GB, that two percent eat 5 to 20 GB per month, and 1 percent consumes tens or even hundreds of GBs. While Virgin Mobile could cancel such accounts, it's not a reliable way of restricting usage and causes hard feelings. Virgin Mobile also certainly did not want to put in overage charges because it's a fully prepaid plan.
Because Virgin Mobile was the lowest cost, I'm also assuming heavy-data users, being mauled with overage fees from Verizon Wireless or Sprint/Clearwire (on the 3G side of the 3G/4G hybrids they offer now), migrated to Virgin Mobile.
Virgin could change the plan's name to "5 GB or 30 days, whichever comes first," which would be like AT&T's iPad plans. But it's perhaps a little kinder than that, offering the throttled rate so you're not suddenly cut off or having to pay the meter right away.