This is a question that comes up from time to time.
Do you side load apps (install from locations other than the Android Market)? Do you try to find *free* versions of commercial apps online and install them?
If your answer is "no" then you aren't in a big at-risk group, but it's not a bad idea to have something.
Lookout Mobile Security is pretty popular I believe that Jerry Hildenbrand from this site uses it.
Anti-Virus Free is offered from the AVG group as well as a paid version.
Much like a PC, Anti-virus software may slow your device down, but with more and more personal information stored on smartphones, it makes more sense than not to consider security.
Do a site search.
This topic has been beaten to death.
I don't use one, but I think Jerry uses it for remote security then for antivirus, but I could be wrong.