As I use AVG on my PC I thought I would try the free Andoid version. I get the impression it is still really in beta as many of the tools on the menu are beta versions. But the basic scan function seems okay, doesn't take very long, only a minute or two (there aren't that many files on Android, not like Windows, last time I looked there were over 1,000,000 files on my Windows 7 system!). It did not find any viruses, but it did recommend I turn off USB debugging (needed for rooting) and to remove z4rootmod. Also it seems to scan every download. Whether it is any good at finding actual viruses is hard to say, I am kind of relying on AVG reputation to assure me of that.
Not much of a recommendation I know, but at least if you want to try it, it doesn't seem to do any harm!