I suppose the correct answer is that anything that connects to the internet needs protection.
Up to now, the relatively low numbers of devices compared to PCs and the low level of confidential transactions on tablets has meant that the bad guys (aka the hackers) have paid little attention to phones and tablets. Also, Apps have been traditionally downloaded from an Apps Store which adds to the protection.
However, as the number of Tablets being used for internet access and in particular online banking increase, so will they become an increased target for hackers.
I guess only a small proportion of tablet owners actually have antiVirus installed at this time. This is probably because most users think they don't need it. At this stage I admit that I don't have any AV on my Tablet S. OK, I don't use it for anything sensitive but I think the time may be fast approaching when we all need it!
There are some good free programs available. I can't recommend one but I personally am looking at AVG Mobile Free and Lookout Mobile Free.
Android app markets (market, getjar, etc etc) have been less controlled than iphone or windows phone app markets.. so it was ineitable that badness would creep in.
But I'd be more worried about apps that swipe your data or personal info than apps that would damage the o/s. Really, you should read carefully what permissions an app asks for before installing it. Whay a game would want access to you contacts list ? For example...