I think that's fair. However, depending on how long such a bus trip could be, a number of routes out of Houston become viable including:
IAH-VCT (Victoria)
IAH-CLL (College Station)
IAH-AUS (a more distant possibility, admittedly)
IAH-SAT (ditto)

CLE-ERI (Erie, PA)
CLE-CMH (Columbus, OH)
CLE-PIT
CLE-FKL (Franklin, PA; a more distant possibility)
CLE-DTW (probably at the outer limit of how far a bus should travel for this purpose)

Admittedly, I posted this topic in the Continental forum because Continental seems to be the only major US airline currently operating such a bus connection. I think an airline that may serve more utility from this would be US Airways, since it has a high concentration of routes in the more densely-populated Eastern Seaboard.

There are numerous routes they serve that have the potential to be served by buses including:
CLT-GSP (Greenville/Spartanburg, SC)
CLT-AVL (Asheville, NC)
PHL-ABE (Allentown, PA)
PHL-MDT (Harrisburg, PA)
PHL-BWI
PHL-NYC (I don't want to suggest that bus service on this route, particularly due to the vagaries of traffic in NYC and the high concentration of business travelers which use the route, should replace all air service. However, it might be an attractive option for some budget-minded travelers)
PHX-TUS (Tucson, AZ)
PHX-FLG (Flagstaff, AZ)

Delta offers us another interesting example:
NYC-BDL (Hartford, CT; at the very upper limit of the length of a bus route)
NYC-PHL
CVG-IND
CVG-DAY (Dayton, OH)
CVG-LEX (Lexington, KY)
CVG-SDF (Louisville, KY)
CVG-CMH (Again, a bit on the longer side of things)
SLC-RKS (Rock Springs, WY; ditto)
ATL-BHM (Birmingham, AL; ditto)
ATL-CSG (Columbus, GA)
ATL-CHA (Chattanooga, TN)
ATL-AGS (Augusta, GA)
(This list excludes routes that are possible with Northwest added to the mix)

I'm not trying to suggest with this post that buses will somehow make turboprops disappear. Regional aircraft still have a purpose. I contest, however, whether routes that are so short, they're really within driving distance, are worth the cost to airlines and consumers, as well as the added burden on the ATC system. Might there be a more efficient way of doing this, and might airlines be able to profit from it?