The law is changing in September, such that you will also be required to pay it if you use iPlayer - however, just having a system that can use iPlayer does not mean you have to pay it - it will be payable only if you use it to watch either live or catch-up TV on iPlayer (other catch-up applications such as ITV's or Channel 5's are not included in this). For some reason, the BBC has decided that either adding a login system or monitoring IP addresses that connect to iPlayer is not the way to go and their official line is that they will continue to monitor usage in the same way as they currently do.
There are many long arguments about whether the TV license vans even exist or if they use radio emissions from devices to monitor what you're watching - some contend that flat screen TVs do not produce sufficient emissions, but there is research that says otherwise.