From Instructions for Form 1099-Misc: http://www.irs.gov/instructions/i1099msc/ar02.html#d0e900
Self-employment tax. Generally, amounts reportable in box 7 are subject to self-employment tax. If payments to individuals are not subject to this tax and are not reportable elsewhere on Form 1099-MISC, report the payments in box 3. However, report section 530 (of the Revenue Act of 1978) worker payments in box 7.
Examples. The following are some examples of payments to be reported in box 7.
Professional service fees, such as fees to attorneys (including corporations), accountants, architects, contractors, engineers, etc.
Fees paid by one professional to another, such as fee-splitting or referral fees.