Article from Talking Points Memo:

A London-to-Washington flight was diverted to Maine on Tuesday when it was discovered passenger Yusuf Islam – formerly known as singer Cat Stevens – was on a government watch list and barred from entering the country, federal officials said.
….
One official, speaking on condition of anonymity, said Islam, 56, was identified by the Advanced Passenger Information System, which requires airlines to send passenger information to Customs and Border Protection’s National Targeting Center. The Transportation Security Administration then was contacted and requested that the plane land at the nearest airport, that official said.

This is seriously fuct up. I sincerely wonder why he was denied admission. This AP article did not mentioned Yusuf Islam‘s citizenship status. I mean would I lose my citizenship if I went to Saudi Arabia became a Muslim and decided to come back home to the USA? Or was he always a foreign national? Regardless, his denial of entry is interesting. I really want to why?

UPDATE:
Apparently he was on some government watch lists. My guess was that he donated money to some charitable organizations that have since been linked to terrorist activity.