You'll need to have the disc in the system to play it.
This is standard for games on any console that has a disc release. You always have to have the disc in to play, otherwise you could just buy the game, install and then return the game to the store.
That was the case for FFXI on the XBOX360 because it was Microsoft's Policy. On the PS2 the discs purpose was just install/reinstall so you were not required to have the disc on the system to play if the software was installed. The disc could be used to boot up the game faster after a change on the POL install/boot mechanism when CoP released.
Still I am not sure what will be SONY policy for this with the PS3. But judging from how it works with the PS2, there's a good chance the disc will be again used just for the install.
Also, the BD-ROM drive in the PS3 is slow *** enough to make them want to avoid using it like plague, on a "fast paced" MMORPG game. (sarcasm intended)