Boost version, Virgin Mobile version) that reads:
Effective 5/6/14, if you reach 2.5GB of combined 3G/4G data during your monthly plan cycle, your data speeds will be reduced to 2G speeds for the rest of that monthly plan. During this time, you may experience slower page loads and file downloads, and lags in streaming media. Data speeds will return to normal as soon as your next monthly plan starts. If you'd rather not wait for your new month to start, you can restart your plan immediately through My Account.
How will I know if my data speeds have been reduced?
If you are close to reaching 2.5GB of data in a month, you will receive a text message alerting you. Once you reach the 2.5GB limit, you will receive a text message letting you know your data speeds will be reduced for the rest of your plan month. You can check how much data you've used at any time during the month by logging into My Account and checking Data & Web history.
By putting this data speed reduction in place, we're making sure we can deliver the same quality of service to all of our nationwide customers – we appreciate your understanding at this time
The Virgin Mobile site's Frequently Asked Questions section also has pages showing the new and old throttling polices.
Throttling itself isn't new on Boost and Virgin. Since 3/23/2012 on Virgin and 1/20/2013 on Boost, speeds have been throttled after 2.5 GB in a month. What's different is that current policy sets the capped speed at 256 Kbps or less. What's changing May 6 is that the throttled speed is described as "2G" rather than 256 Kbps. On CDMA networks like Sprint's, 2G means 1xRTT which has a theoretical maximum speed of 144 Kbps and typical real-world speeds of 60-80 Kbps. I'm sure Sprint won't actually switch you to 1xRTT, which uses spectrum less efficiently than 3G EVDO or 4G LTE, they will just slow the data down at the network level.
We probably won't know what the actual throttle speed is until the new policy goes into effect, but I suspect it will be either 128 Kbps or 64 Kbps. Boost and Virgin's 256 Kbps throttle was actually fairly generous, most other operators and MVNOs that throttle already do it to 128 Kbps or less. 256 Kbps wasn't much slower than typical Sprint 3G and was fast enough that web browsing was doable and streaming music services like Pandora still worked reasonably well. 128 Kbps or 64 Kbps will make browsing very slooow and music buffer badly.
The Virgin Mobile FAQ page also indicates that customers who have the extra-cost tethering add-on that was available for a few discontinued WiMAX phones will continue to receive 3.5 GB of data instead of 2.5 GB before throttling to 2G speeds kicks in.