After outlining plans and pricing for PlayStation Vita data in Japan, NTT Docomo's recent press release concludes with a section called "In addition" that describes the preset browser filter against "harmful" websites and how to get it taken off.
If it's the case that you'd like to remove the "prepaid filter," you need to visit a Docomo Shop in person to present your Docomo UIM card and a piece of ID that proves your age (such as a driver's license, etc). Furthermore, if you're under 20 years old, you'll also need a letter of consent from a parent.
Otakomu dug up this factoid, and their headline pretty clearly reflects their opinion on the matter—basically, "If you want to see porn sites on your PS Vita, you need permission from a Docomo Shop lady." Kind of makes you wonder how many people will go to the trouble.
Previously on Crunchyroll News: PlayStation Vita Launches in Japan December 17th