There's an even better plan for those who plan on taking their Vita online frequently, as shown in a pamphlet promoting Vita 3G plans.
For ¥5,460 a month, you can get unlimited data through your Vita. This is the same "FOMA high speed" access that's included in the 3 hours bonus time with the 100 hour pre-paid card. It offers 14Mbps download and 5.7Mbps upload, far above 3G's 128kbps/64kbps speeds.
The plan will also be available as a sliding ¥1,000 to ¥5,985 plan, where the base fee is ¥1,000, and you get charged for use until you reach ¥5,980, where you can continue with unlimited use at no additional charge -- this is a typical option for data plans.