
The reasoning behind the strategy is that dropping the PS2 price would disrupt the Xbox 360 launch. In that scenario Microsoft would be selling both the Xbox (at $99) and Xbox 360 (at $299.99?) at loss, while Sony would still be making profit with PS2.
It is quite unlikely that Xbox 360 would do so bad at launch that Sony could afford delaying the launch of PS3. Then again the plan could save Sony alot of money. It is estimated that Sony could lose more than $1 billion on PS3 hardware.
Source:
GameSpot