mwhahahaha...I laugh in the face of your puny sandbox!
It's absolutely false that the iPhone/iPad is totally secure. Nothing is secure that can access the internet, or that another person can easily access. Heck, your biggest risk is probably a coworker with a grudge! (If this is a concern, you should turn on passcode lock and encrypt your device. Probably not sufficient against a really determined person, but an adequate deterrent for the great unwashed.)
It's relatively easy to crack a browser open by passing it a malformed image or other item for manipulation/rendering.
In addition, any application that creates its own server or manipulates data on the web provides an additional vector of attack for potential hackers, while the app is running anyway.
Perhaps some of you remember the
SMS virus that plagued the iPhone not too long ago? This virus worked on all iphones, not just jailbroken ones, by sending a malformed text message to all your contacts.
Doesn't matter though, because....
Firewalls won't protect you from these attacks anyway. An antivirus checker MIGHT, but the odds are very slim that it would catch it before it did serious harm or passed itself to the next recipient.
Don't let yourself get all knotted up over security. NOTHING is secure. One of my favorite profs used to say that encasing a computer in a block of concrete and dropping it down the Mariana trench wouldn't make it secure!
It is important to note that Apple is, for the most part, doing its due diligence on security for these products. You should expect that the average person will have zero problems with viruses.
If you are doing sensitive things for your work on your iPad, I would consult your company security policy for what they consider adequate protection on a device like this.
But the end of the story is that most users will likely find the iPhone/iPad considerably more resistant to tampering than their other computing devices. Just remember to "never say never". False confidence is never a good thing.