One virus with low level access to the hardware spreading across homogenous hardware and software (cell) - that's designed to talk to each other, no less - could bring down the whole system (i.e. a network of cell computers, beit worldwide or whatever). When he talks about a "worldwide system", he means cell computers on the internet - obviously not all computers. If you have a closed box, the same hardware, and one virus can exploit a vulnerability in that hardware, then a lot of systems can be exposed. The concern is not without merit.
The thing is, Cell can be coded at a low level (but there is hardware protection and security that may make it difficult to execute malicious code).
Perhaps the concern is regarding the wider development community, since Cell is being made an open platform (and for example, with Linux set to be widely distributed on PS3) - and the normal problems with that follow (e.g. virii). Perhaps low-level access to hardware will be restricted for the regular folk who pick a PS3 or Cell computer, or whatever, off a shelf, but for licensed devs (game developers), low level access is A-OK. Who knows.