RFID viruses could only work if you create a special back end system that is designed to allow itself to be corrupted. (And we all try to develop those kinds of systems, right?)
License plate RFID tags (like companion animal ID tags -- e.g., "Does Your Cat Have a Computer Virus?") will have only a simple number in them. To "infect" a computer system, you'd need 1) a lot more memory to carry code and 2) a system stupid enough to accept anything other than the number as anything other than a database entry. In fact, the system would have to be stupid enough to accept unexpected data and move it up to the enterprise software "just to see what happens."
Now, here's what happens in real life. Imagine you're typing your phone number into a database field. Instead of your phone number, you try to input a program. The database will accept only numbers and only the right number of them. In other words, it will ignore your attempts to do anything except enter the correct number of digits. That's what any decently designed (not even really smart) RFID system will do with unexpected data. Ignore it.
In fact, it's exactly what happens in this space if you try to input HTML tags. They simply don't work. Neither will these so-called RFID Viruses.
So, don't believe in this imaginary threat. Remember, a small amount of common sense will overcome a whole lot of stupid.