I remember trying to replace a T1 with Comcast Business Cable. It was faster in the office, but variable latency, dropped packets, less usable remote access (just ssh and scp) and more frequent maintenance outages killed it.
IIRC, SDSL turned out to be the better answer (and we really, really wanted to try FiOS, but they didn't support our building).
Has anyone tried some kind of bonding between the T1 and cable? A reliable but slow connection and a fast but variable connection seem like they could be much cheaper than a fast and reliable connection.