It's theoretically possible for someone to write a virus that attacks Pandora. But in practice it's unlikely. They'll focus on the most popular platforms -- I expect Pandora will be way down the list.
Even a virus that attacks Linux might not attack Pandora -- because it will have been written for the x86 Linux platform and won't be compatible with the ARM Linux that'll run on Pandora. Viruses for another ARM Linux platform (if such viruses ever exist)... might work on Pandora.
A lot of discussion goes on here about what it'll take to get a given piece of (game or emulator) software to run on Pandora. Well, a virus is software too. If nobody makes the effort to get it to run, it won't run. And for a virus to do its thing, it has to not only run, it has to run well, without help from the machine's owner.
It's theoretically possible to make a cross-platform virus. It's more difficult and therefore I expect it's less likely.
If you run an emulator of some popular system (dosbox) then it would be vulnerable to viruses, but they'd probably be trapped within the emulated system and not be able to do much harm there.
The only other reason you might want an antivirus on Pandora is to clean up files that are destined for some other machine (for example, suppose while you're browsing the web on Pandora you see a game you must have for your PC, and download it). But having antivirus on that other machine would solve that problem.
If there are opensource antivirus programs (I have to admit, I don't know whether there are) they could be ported. Would they? I doubt anyone would make the effort if there aren't viruses.