Frank Williams said:
Kazuki Nakajima said:Kazuki has consistently demonstrated his potential through karting and other junior Formulae. Having assessed both his potential in the car and his aptitude out of it, we believe he has the necessary ingredients to graduate to Formula One in a testing role.
Yoshiaki Kinoshita, General Manager, Motorsport Division, Toyota Motor Corporation said:For anyone with an interest in motor racing, Formula One is the ultimate objective and I am delighted that Williams, one of the senior names in the sport and a team that exists solely to race, has given me this great chance. I look forward to helping as much as I can with the team?s transition to Toyota power for 2007. It will be a great honour to be the first TDP driver to graduate to Formula One.
Kazuki Nakajima is the son of Formula One driver Satoru Nakajima, who raced for Lotus and Tyrrell in the mid-80?s and early 1990?s. His best result was 4th in the 1987 British GP and the 1989 Australian GP.We are extremely delighted that Williams has chosen one of the drivers from the Toyota Young Drivers Program as an official team test driver. It will be certainly a big opportunity for Kazuki and we hope that Kazuki will make a contribution to Williams' test program and their fight back in the championship.
How many test drivers do they have at Williams? I sure there are at least half a dozen. 
First of all he lacks the experience to catch the others and crash into them. Sato and Monteiro can accomplish that during testing. Secondly, he needs to prove himself against Karthikeyan. Surely Nakajima has potential (otherwise FW wouldn't have signed him) but being 21, he has a few years in front of him before he can accomplish a big feat.