Your question is answered on the SSA website:
http://www.socialsecurity.gov/disabilityresearch/wi/generalinfo.htm#work
"How do we evaluate your work activity for SGA purposes?
We generally use earnings guidelines to evaluate whether your work activity is SGA, and whether we may consider you disabled under the law.
The amount of monthly earnings considered as SGA depends on the nature of a person’s disability. The Social Security Act specifies a higher SGA amount for statutorily blind individuals. If your impairment is anything other than blindness, earnings averaging over $940 a month (for the year 2008) generally demonstrate SGA. If you are blind, earnings averaging over $1570 a month (for the year 2008) generally demonstrate SGA."
SGA=“substantial gainful activity”, used to describe a level of work activity and earnings