You are right. Hoglahoo did a field test (which almost cost me a green zit), and the result is as follows:
- Thanks adds one point to your rep.
- Positive rep adds as many rep points to THEIR rep as you have rep power (go to the main page, your rep power is in the upper right hand corner of your user cp field).
- Negative rep subtracts half as many rep points from THEIR rep as you have rep power (rounded up).
Quite simple, really. And I find it quite useful, too. Except when it's misused in order to harass users.