[Edit: Since originally posting this article (from the Sunday Times newspaper) a number of stringers from the Wimbledon stringing room and other events have said that they have never seen anything that would suggest that pros use silicome spray on their strings. Whether that means that they spray themselves after the racquets have been strung or the article is BS no one knows. Nevertheless you should treat the article below with a healthy dose of scepticism since it does not quote anyone or evidence that which it alleges/speculates in the article. Just because a story is printed in a newspaper does not mean its true.]
They definitely need to ban the application of external substances (other than logo paint) on strings. Doesn't say who the culprits are though and what type of strings this is being used on....
The Sunday Times
Barry Flatman Published: 24 June 2012
A Rafa Nadal topspin shot can rotate the ball at up to 4,900 revolutions a minute (Paul Childs)
TOP tennis players are hugely increasing the spin on a ball by secretly spraying their racket strings with silicon.
Former tennis stars, including two Wimbledon champions, this weekend called for a crackdown on the use of the spray. Pat Cash, who triumphed at Wimbledon in 1987, said: “The game’s values are gone.” In the 1960s, the Australian Ken Rosewall, one of the finest exponents of spin, got up to 2,000 revolutions of the ball per minute. Today’s players can reach 4,900rpm.
Cash said former players including Mats Wilander, John McEnroe and Ivan Lendl were supportive of his call for outlawing the use of silicon. Cash said Lendl “maintains string technology has gone way too far” and that the use of silicon should be banned.
Modern racket strings stretch and snap back in a fraction of a second, increasing the spin on a ball. Players enhance that spin further with silicon.
The International Tennis Federation is monitoring the situation but has taken no action.