I wonder if it'd be possible to spoof it as a Wii instead, since the Wii never receives anything more than 480p.
Alternately, what Dark-Show says might be right, and maybe not even for copy protection purposes. It may be possible that they switched over to MPEG-4 AVC (H.264) encoding to save on total bandwidth. All modern devices that support Netflix have no problems decoding AVC (even the Wii), but I seriously doubt that the PS2 has the horsepower to do so. So, even if you could get Netflix to serve video to the PS2, it might not be able to decode it.
Chip-free/MC-free homebrew <3
FMCB 1.93 on MC for emergencies, NA
250GB HDD/FHDB 1.93/OPL 0.9.2 (GSM and non-GSM)/PS2 games, NO HDDOSD
80GB HDD/FHDB 1.93/POPStarter rev12/OPL644/GSM0.37/PS1 games, NO HDDOSD
SCPH-75001 (sitting idle, slims suck):
FMCB 1.92 on MC