Nope, should be straight forward.
Turn off your modem/STB and get your Mac up and running (leave registering to later). Turn that off. Then power on the modem/STB, wait til the lights have settled and then power on your Mac. If you are using OSX (Tiger, Panther, Jaguar, anything 10.x) you should be fine, it should automatically detect and sort it out.
EDIT:Once you turn your Mac on and detect the internet, if you have a STB you have to re-register at
http://start.ntl. Once you you do that, (you have to do it within 5 mins of STB bootup), you have to restart to get your proper IP (not a 10.x.x.x one), otherwise your connection will die. I don't think you need to do this for modems.