Why do you feel the need to anti-virus?
It's not needed....
I think it is need for any phone that have a browser. My Storm 1 got a Virus that drains the Application Memory. I can not fix it at all. I did all I can do including took it to Verizon and still I can't get it to stop Draining the App memory. I am waiting to get the Droid X when all the kinks and bugs are out of it. I learned from the Storm never to get a phone when it first comes out. I will never do it again because of all the problems I had with the Storm 1 self rebooting, freezing,etc... If they have anti-Virus for I will put it on the phone. Especially if you have downloaded music on it.
Virus? Doubt it. More likely a rouge app.....A factory reset will fix it.
Been there did that Factory reset. Still draining my Memory. Trying to hold out a month or 2 for the Droid X.