Viruses are usually built for Microsoft Windows, there are a few for cellphones and for some other computer OSs but they don't come close in quality and amount to the Windows ones... Now, consoles like the Wii or the PS3 and handhelds like the DS and PSP have hacks but not a single real virus...
Now the 360 is considered a special case because is made by M$, but the truth is that currently the 360 has no virus because, like the PS3 and the Wii, they have their own architecture and OSs (that are not Windows), so the viruses have to be custom made for the console, then there's the Network case, everything that works with the console's OSs is official and signed. The hacks and homebrews can change this but when you hack a console you're on your own, and to do it you have to know what you're doing.
In conclusion, you don't have nothing to worry about for now and the near future...







