And Cisco only uses linux on its VOIP and consumer products. IOS (found on its enterprise firewall and router products) is a hardened operating system and the backbone of the Internet core routers.
Actually, Cisco PIX does not run "IOS". The IOS that runs on Cisco's "enterprise" class routing platforms is still using (unprotected) "shared" memory architecture and is constantly plagued by bugs. These bugs are often triggered by certain combinations of features and can turn an upgrade into a nightmarish game of Russian roulette.
However, IOS-XR eliminates the majority of these problems but is only available on a handful of high end carrier class routers (GSR, CRS-1, etc.).