The anti-virus tools in Kaspersky Mobile Security seemed to work well. After you install the app it recommends scanning the entire contents of your phone. This took 50 minutes on my Samsung Galaxy S, but the amount of time didn't matter. The app used between 2.5% and 13% of my phone's CPU, which left plenty enough crunching power to make calls, send emails, tweet -- even watch an HD movie trailer on YouTube.
After the initial scan you can choose from three modes: extended, recommended or disabled. "Recommended" mode worked well for me. The app quickly scanned all the new apps I downloaded, and the background operations did not cause a noticeable performance slowdown or even reduced battery life.