Michael R. Freeman , Jul 19, 2009; 09:44 a.m.
TTL mode will *not* work. If you attempt to use TTL on the SB-15 with the D90, the camera will refuse to fire the shutter.
The non-TTL auto modes (A1, A2) and the manual mode (M) will work.
There are much better choices for a D90 flash, but you will do no harm or cause any problems by using one.