It depends on if it has a CableCard slot. If so, your cable company "rents" a card to you that you install in the Xbox (similar to a PCMCIA card slot on a laptop) and boom, subscription cable TV.
Either that or it has a QAM tuner in it and it can only get stuff transmitted in the clear. Would probably be a good idea to slap an ATSC tuner in there as well if they're going for the "one-for-all" approach.
Although, if this thing doesn't have DVR capabilities, then what's the point?