There's something to do with how games are pressed onto a Blu-Ray disc rather than them being signed or not, as I recall.
As an example, burn a PS3 game to a BD-RE (which the PS3 can read), put it in and then try and load it through a backup manager. It won't work. If you browse around your PS3, you'll find that under Video, it recognises that it's a BD-RE data disc, but can't do anything with it.
So I could be wrong, but I think there is more going off than just making signed discs.
60GB (120GB WD Scorpio Drive) PS3 (CECHA01, NTSC), 3.7OFW
60GB (640GB Samsung Drive) PS3 (CECHC04, PAL), Kmeaw 3.55, Repaired (Dead Laser)
PSP-1003 FAT, CFW5.00 M33-6 & Prometheus v2
X-Box 360 Xenon (JTAG'ed, 500GB HDD)