I'm not so sure that will continue with Apple's embracing of the Intel processor. Granted, dll-loading and system API calls within the virus code designed for Windows will not work, but I'd still think hackers could now have the capability to write platform-agnostic viruses for Intel that could do a phenomenal amount of damage. Your thoughts?
If the attack is designed in machine code, it probably won't be very portable. But it's possible to use cross-platform scripting and network vulnerabilities to attack different operating systems, even with different CPU architectures.
As far as I know, viruses are not written for processors... they are written to exploit flaws in operating systems. There have been a few viruses aimed at BIOSes, but none that are aimed at specific processors. For example, most Windows viruses work just fine on a PowerPC chip based Mac running a Windows installation in VirtualPC yet the PowerPC is completely different from the Intel/AMD X86 design.