It's the same situation as Desktop/Server Linux - technically, it's possible but practically it's less of an issue than we're used to with Windows machines. The permissions system mixed with the way that we install programs (authorised repositories as opposed to 'that file you downloaded') creates a much more secure device, and in the FLOSS world the effort tends to be directed toward patching security holes (or making sure they don't come up in the first place) rather than reactive 'anti-virus' software. The best protection against malicious software is to stick to the approved and properly-administered repositories.

Others might (will probably) disagree, but I think one of the strengths of Linux (and other UNIX derivatives) is the focus on proactive security, negating the need for reactive defenses. Locking the door before the horse bolts, so to speak

Edit: Also, sticking to well-maintained Open Source software is inherently more secure - not that I'd want to take anything away from the proprietary software available in Ovi Store