Yup, part of the upcoming DOCSIS 3.0 standard is Rumored to inclue some type of channelbonding scheme. Allowing use of more than one QAM carrier at a time. I believe that both cisco, broadcom and motorola have submitted plans to CableLabs.
Where/how this bonding will occur is still in question - hence the proposals to CableLabs. I have seen bandwith up to 100 Mbps and above mentioned as posibilities with this technology.
I believe this can be utilized on both the up and downstream carriers.
Nicholaus