I agree with you that the DC suburbs should be placed in the Northeast corridor. However, as a resident of this area of Maryland I can say the reason the author probably placed Prince Georges County (bordering northern and eastern DC) in the Southern Lowlands is that it is a predominantly black county. Even though it is a reliably Democratic area and looks more like the Northeast corridor in that sense, the Democratic votes are not coming from liberal whites in the same way as the rest of the corridor. Besides that, the author missed Arlington, Alexandra, and Fairfax county. The former two round out the DC box that was originally area ceded from Virginia to DC but later returned to the state. To be fair, Fairfax county looks more like a high-growth southern county than a member of the Northeast corridor; it is kind of a hybrid of both. It voted Republican solidly and probably still will given GWB's strength... only in the 2003 elections did a shift towards Democrats become apparent, perhaps as the area becomes more developed and affluent.
Overall, quite an impressive site.
