SirNickity:
Just an idea.
A good idea.
I seem to remember reading something in the datasheet about whether the ISP polls for completion of a flash operation, or just waits until "probably" long enough before sending the next command.
Most (all?) AVR processors in the Arduino world support polling. It is much more effective than waiting. But, the ArduinoISP sketch does not poll (at least it didn't that last time I checked).
I believe the ArduinoISP sketch waits approximately twice the time Atmel recommends which is about twice again from the time programming actually takes. But, as SirNickity stated, it is worth checking the delay to ensure it really is long enough.