I see two problems with the "Open Letter".
The first concerns its form. It is addressed to something called the "left establishment", but (and this is indicative of the kind of left we now have) few of these individuals hold positions of public or mass-organizational responsibility. They are editors, writers and academics who may be leading exemplars of a certain current of opinion, but represent little more than themselves. The appropriate response would therefore seem to be to polemicize against them, not petition them for redress.
On content: The letter takes these people to task for their support of Obama, but fails to mention the larger problem, namely, the historic attachment of them, and so many who think like them, to the Democratic Party as a whole. This is the strategic problem for the left, it seems to me. The mere repudiation of a single Democratic president or candidate fails to address the institutional role of the DP. It still leaves the way open for left-liberal Democratic primary challenges like the Jackson campaigns of the 80s. The problem with such primary campaigns is that the challengers remain party-loyal, and therefore back the centrist standard bearer when they (inevitably) lose. The dissidents thus ultimately function as the left face of a party completely dedicated to the rule of capital, useful to the leadership for keeping left or potential left constituencies within the fold at election time, but roundly ignored in between. It is necessary not only to repudiate Obama, but to break with the Dems and the lesser-evilist psychology that keeps the left in a state of paralysis. If Feingold or Kucinich want to be truly progressive, let them run as independents.
Jim