Police in the UK, working in partnership with the FBI, have arrested a 19-year-old UK citizen, Ryan Cleary, alleged to be a member of Anonymous and LulzSec. Ars Technica's Peter Bright (@drpizza) has more on the arrest.
LulzSec, for its part, is denying that any member of their group has been arrested, wondering which "poor bastard" had been taken in. However, their IRC server is offline, which would be consistent with claims that it was operated by Cleary.
The U.K. police and the FBI say they have seized a significant amount of forensic "material" from Cleary's address. LulzSec also denies that it was responsible for posting claimed U.K. census data earlier today, pointing out that anyone can falsify LulzSec-style press releases, and that it would announce any operations on its Twitter feed.