This really depends on where you live.
Some systems place High Pass Filters and/or Traps on a customer's line if they are not an active HSD customer. This prevents the cable modem from being able to talk back to the head end and/or from receiving a cable modem signal.

If this is the company's ONLY way of protection, and they forgot to put one on your line, then you may be able to hook up a cable modem and surf away.

This is unlikely though.

Most cable companies now authorize your cable modem by serial number and/or MAC address. Attempting to use a non-authorized modem will just end up in your modem being denied access to the network.