I saw an announcement on the wall at Matiatia that the 40-ride tickets will get a time limit of 12 months' validity after purchase. The old style tickets will expire in March 2011. The reason given by Fullers is to make the tickets tamper-proof, although I have not heard of any incidents of people boarding with falsified 40-rides.
This is just a fare increase by stealth as it will force occasional travellers (i.e. those making fewer than 40 trips a year) to buy an non-time limited 10 ride ticket, which is more expensive per ride.
Oh, and calling it a "Flexi-pass" is a misnomer as it actually decreases flexibility.