Cisco PIX 515-->520?

[Todd Suiter <todd () s4r com>]

I'm hearing rumors of problems with the 515 series PIX:

'...that in some cases
Cisco has opted to replace customers'  515 Pixes with 520's at no charge
because the 515 in some cases accepts packets with spoofed source
addresses that it should be able to reject.'

has anyone heard of this? Far as I know the sw is the same, but hw is different.