I run AVG. So far the only threat it has found is when I leave the allow 'unknown sources' ticked in settings and once when I ran a script from XDA, (which they explained).
I run AVG on my PC so I guess out of habit I installed it on my tablet, but hard to say if it is necessary.
I did a factory reset after updating to ICS because of problems I was having. It didn't fix the problems and I wouldn't say it is necessary.
On the other hand, unless you have a lot of apps, passwords and accounts it is not a serious pain to start fresh and can't hurt.