The announcement is brave, but is also an extremely savvy move by the State Department. By describing the already-existing contact, Secretary Clinton is preemptively framing future MB engagement as consistent with previous US policy for a Congress who is decidedly against engagement. Given that "limited contact" is not an objective standard by any measure, the State Department also has considerable latitude to increase its engagement while still claiming that overall contact is limited.
For US strategic interests abroad, the move is crucial. Without mandating any concrete policy change, it takes the wind out of the sails of radicals in the Muslim world who claim the US is at odds with Islam and with the Brotherhood. It also opens up opportunities for the US to exert influence over what Islamist participation in the new Egyptian government will look like.
Most importantly, the move demonstrates to Egyptians that the US is responding proactively to the events of the Arab Spring. It shows that the US is trying to shape the kind of nuanced and complex engagement that has been lacking in the past. The move also lends credibility to US claims of democracy promotion since it is engaging with all Egyptian political parties rather than picking and choosing.
US engagement with the Brotherhood should certainly be deliberate, but limited contact meets this standard. Ultimately, the benefits of promoting US security interests far outweigh the cost of lending slightly more legitimacy to the Brotherhood itself.