From the best we can determine, the Virgin Mobile unlimited data plan is using the Sprint Pre-Pay foot print - which is basically their non-data roaming areas. Fantastic price if you need access mainly from metro areas.
However, if you venture outside of metro areas and need access from places that Sprint roams to (which is a lot of the country, btw) - you may want to look into Millenicom's Sprint plan for $69.99/mo. It has a fuzzy 20GB cap per month, but does not have roaming limits (unlike a plan directly with Sprint which has a 300MB roaming limit - even on their grandfathered in 'unlimited' plans). They also have a Verizon plan for $59.99/mo with a 10GB hard cap.
- Cherie