It is already split once if not twice from the outside cable box to the various places in your home. You should definatly be fine.
I had questioned this myself a few weeks agao, since I was going to hook a media center up in my office to experiment with the DVR function of it. I need to have a cable going into the tuner card to do video capture and set the programming guide.
I of course have the cable line in there hooked to my cable modem for the Internet.
I went to my local radio shack and got a standard spltter. The guy there told me that it might decrease the cable modem speed and decrease the tv signal quality. He then went on to say that monster cable is coming out with a splitter that promises to allow this to work without any data loss or picture quality difference.
I laughed at that.
Basically, the way that cable tv and the cable Internet works is by operating on channels, spread over the frequency that is designated by the cable company.
I think knowing how stuff works is really interesting; also knowing how it works is a good way to help you diagnose problems when they arise.