All of Virgin Mobile's beyond talk plans have unlimited data, but once it goes past 2.5 GB at 3G speeds, it will be throttled. This means that you can continue to use data (surf the web, email, apps, etc) except the speed will be slower.
I believe you have mad a misconception. The 2.5 GB is a data cap for heavy data users. You see, everyone can get an unlimited data plan. However, when a user exceeds 2.5 GB of data, VM throttles the data speeds for their account. They can still keep using data but, it will remain throttled until the next month when the cap is renewed.
Tough crowd here. Virgin Mobile answered your question in the FAQs:
Reduced data speed on Beyond Talk plan Q&A
Q. When and how will data speed reduction affect my Beyond Talk Plan?
A. Starting March 23rd, 2012, if you use over 2.5GB of data during your monthly cycle, your data speeds may be reduced to 256kbps or below for the rest of that monthly plan cycle. 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 plan month starts. If you'd rather not wait for your new month to start, you can restart your plan immediately through My Account.
Q. How will I know if I hit 2.5GB?
A. If you reach 2.5GB of data during your monthly cycle, your data speeds may be reduced to 256 kbps or below for the rest of that monthly plan cycle. If your data speeds are reduced for the rest of your plan month, you will receive a text message notifying you of this change. If you are susceptible to data speed reduction, 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.
Q. How much data is 2.5GB?
A. To give you an idea, it's about 400,000 mobile internet page views, about 90,000(!) emails (without attachments), 91 hours of streaming music, or 20 hours of video clips.
Q. I have a HTC EVO V™ 4G and I purchased Mobile Hotspot access. Will I still experience data speed reduction when my overall data usage reaches 2.5GB?
A. No. Purchasing the Mobile Hotspot feature will give you an extra 1GB of full speed. Connected devices, as well as the HTC EVO V™ 4G, will not experience a reduction in throughput speeds until your overall combined monthly 3G and 4G data usage from phone and hotspot usage. Once 3.5GB of data is consumed during the monthly cycle, data speeds may be reduced to 256 kbps or below for the rest of that monthly plan cycle. If your data speeds are reduced for the rest of the plan month, you will receive a text message notifying you of this change.
Your answer is to restart your plan immediately if you've hit the threshold.
DATA is UNLIMITED, the speed is REDUCED after 2.5GB not stopped