Is an antivirus really needed for Android? I use Linux for half a decade now, so I am not used to dealing with viruses anymore. To my surprise, I saw some discussion about antiviruses in Android. I haven't installed any yet, but this bugs me: are they really necessary? Are there (or were) any known viruses in the market? Doesn't Google runs an antivirus before allowing an app to the market?

I haven't had no problem in one year of Android, but my cellphone has a lot of important information, and I wouldn't like to be surprised with some virus cleaning my phone, or making international calls while I sleep, or whatsoever. Should I install an antivirus?
You shouldn't need to worry about running an anti-virus on your phone. I would recommend running one on your PC though and having it set to scan peripheral devices. When you plug in your phone and it gets mounted as a mass storage device the desktop's antivirus will scan the phone for viruses' signatures and you will be doing the minimum to ensure your security. 

It is an illusion to believe that just because you are running *nix or OSX you won't benefit from the security afforded by AV software; that said you dont need to spend anything to get a good, lean program especially in Linux.