There are basically no network exposed services on the NITs by default. If you run a SSH server or similar on the N800, it would probably be a good idea to change the default. In general, there are very few viruses and malware that target Linux specifically, as it is a relatively hard target and there are fewer machines deployed that meet a certain attack profile. Hackers like to go for the low hanging fruit. The biggest risk you run with any device while doing online banking is going to be man in the middle attacks. Be sure to type in your bank's url directly and not via a link sent to you in an email (phishing scam). Check that SSL is being used and that the SSL certificate checks out. I know this sounds like a lot to do, but these are security procedures you should be following while doing online banking from any device, not just the N800.

A brief aside, your bank may or may not work with the default web browser on the N800. You are likely to have much better luck with the Firefox derived browser that ships with OS 2008, which is expected for the N800 "real soon now."