For the basic "virus" concern, Mac OS and iOS (iphone, ipad, etc.) are generally more secure than a Windows or Android device. Apple's products are more secure because they are less accessible, for lack of a better term. As a programmer (including a virus programmer), your programming options are more limited. That's not much of a deterrent these days, but that's one reason they've been less risky in the past (and why many people are more mislead about the current and future risks). With regards to Apple mobile devices and iOS, Apple restricts and allows only what they want through the app store. That's not to say they won't let anything risky through, but overall, it is safer than just allowing any application that is written for Windows. The Windows app store is also supposed to be secured, but really, we have no guarantee with either. Because Windows and Android both are more open, they are more susceptible to viruses. As popularity shifts, there will be more benefit for virus writers to attack the more popular devices.
The bigger concern, however, is not simply viruses. The network you're using can also be at risk. As others have stated, if you're in a public area connected to a Wi-Fi hotspot, there is risk that someone else connected there could be "watching" your traffic. You should only use websites/apps that use encryption when transmitting information - especially banking info. Usually, the apps' descriptions will mention if they use encryption. Also, if you can create a VPN, that could help though you may not have that option most of the time.
To answer your question generally, you should always be concerned with security. Tablets are really no different than PC's regarding the possibility of risk. That being said, an iPad out of the box is relatively safe (for now). If you do use banking apps, be sure to check your transactions often and check with your bank regarding liability in the event of a hack.