My Aunt and Uncle flew back to ORD yesterday via SFO on UA 152 a B757-200. When they were on finals to ORD, the captain informed them that the landing gear may not have been locked into place. There was supposedly panic in the aircraft as a crash landing may have had to be made. They then circled ORD twice and made a flyby. The control tower said that the gear was down but might not have been locked. They then were vectored to one of ORDs remote runways and made a safe landing. A very close call. They said that there was no foam on the runway, as there normally is when a belly landing may have to be done. Wow! I would never have wanted to be in such a situation!