Hi,
I have been installing Cat5 and all it's variations for over 8 years and RayG is correct in what he says, technically, but like you said logically why not, I have seen people using telephone cable for ethernet but only 10mbps stuff, try it and see. But in order to use it for both options you will need to have at least 3 pairs of wires in there (1pair for the phone and 2 pairs for the ethernet), I mention this because in the UK 3 pairs are standard but in the US 2 pairs seem more common. Hope this helps, let us know how you get on.
Regards,
Del