anyone know why the PPS rate just halved (in the last 2 hours)??
The PPS rate quoted on BTCGuild.com is correct. You can calculate it for yourself:
PPS rate = blockreward / mining difficulty * (1 - pool fee)
= 25/4250217919.8695 * 0.925
= 5.440897e-09
PPS rate quoted on BTCGuild: 0.0000000054408975