This is from the IRS site:
Can catch-up contributions be made to a SEP?
No. SEPs are funded by employer contributions only. However, catch-up contributions can be made to the IRAs that hold the SEP contributions if the SEP-IRA documents allow. The catch-up IRA contribution amount (for employees age 50 and older) is $1,000 for 2006 and later years.
Here's the link:
http://www.irs.gov/retirement/article/0,,id=111419,00.html