As Linux becomes more and more popular on the desktop will there be more viruses written for it in the future? As viruses can't spread through a Linux system like they could on Windows (unless infected while in root?) will viruses mutate into something that can get around this or take advantage somehow of the OS to the extent of being more than just an annoyance (which is what I've read recently about Linux viruses)? Or, will linux spell the end of the virus in a future sans Windows?

Spyware and adware are windows-centric as well, but do they occur at all on Linux?

Probably more socially engineered virus to get some dumb fsck to run at root.

you will probably start seeing cross platform first. (i.e. java) but you will see them. I can say with confidence that they will probably not be as destructive in the overall number of systems affected as the windows based are. That statement is made not based on the fact that there are more windows systems out there but that as a percentage of the whole fewer *nix systems will be affected due to a better security model.