According to Ric Bucher of ESPN.com, Rasheed Wallace has been working out with Marcus Camby and Kurt Thomas at the Knicks practice facility, adding that the team is "considering" him (Twitter link).
Wallace has not played in an NBA game since June 2010 and officially announced his retirement in August of that year. This past February, there was a report which suggested that the he was planning to sign with the Lakers and had also been linked to the Heat around March, but neither situation materialized.
The 6'11 forward has some previous ties with current Knicks head coach Mike Woodson, who had been an assistant coach under Larry Brown when the Pistons won the NBA title in 2003-04. Should Wallace be invited to training camp, the recently-turned 38-year-old would be the fourth eldest player on the roster, behind Kurt Thomas (set to turn 40 in October), Jason Kidd (39), and Marcus Camby, who is older by about six months.