It isn't "infecting" a linux machine as such. When the n900 is attached to a windows machine it is just a storage device like a hard disk.

So the virus is on windows and just attaches itself to files on the n900 filesystem.

You could then open those files, even run them on the n900 and nothing untoward would happen.

You would need to plug the n900 into a different Windows machine and execute the infected files on the Windows machine for the virus to propagate. So I guess you could call the n900 a carrier at the most.

A virus is an application like any other, and is designed to run within a specific environment. You can't run x86 code on an n900 so it is immune to x86 viruses.