SocialSecurity.gov has this information about social security tax limits.
The limits for social security go up every year. For employeed individuals it is 6.2% on earnings up to $97,500 for 2007 and will go up to 6.2% on earnings up to $102,000.
According to the Wiki in the US "Once that amount is earned for a given year, neither the employee nor the employer owe any additional social security tax for that year."
Please refer to IRS publication 15 for details.
More searches on this yielded the fact that individuals might see " Social security limit reached" in their paycheck.