The television could introduce noise into the line, but since no matter where the tv hooks up in your house it's connected in the same way(parallel), I'd say you're fine. It won't slow you down if it works(comes online). The modem operates at different frequencies than what your television channels are on. This is what enables the different services to coexist. I have had problems in the past with my cable modem's upstream causing lines in my picture. I've had problems with my modem being on the same drop as my televisions and having to have a new drop put in just for my modem. No matter what the cable company will take care of you. Don't worry. Also, something to be aware of is some splitters use diodes to keep the televisions from injecting noise into the line. You may need to open your splitter and bypass the diode on the lead going to your cable modem so it can send upstream data.
I do this for a living so I have experienced most of the things that can go wrong. (IMG:
style_emoticons/default/smile.gif) Good luck.