From the press release:

The American people, through the nation’s library, will receive a post-holiday gift of vintage sound recordings from one of the world’s largest recording companies. The Library of Congress and the Universal Music Group (UMG) announced today the donation of more than 200,000 historic master recordings—many long out-of-print or never released—to the Library’s Recorded Sound Section, which has more than 3 million sound recordings in its collections.    Universal Music Group Donates Recordings to Library – The Library Today Library of Congress.

Note, though, that Universal retains the commercial rights and the copyright.  So it seems as though the Library will do the digitizing, saving Universal a huge amount of money, and Universal gets to sell any of the results it thinks it can. Given that re-issue decisions are made on a standard cost/benefit basis, it seems that now that a substantial portion of the cost is assumed by the Library, Universal is more likely to try for commercial releases of items previously assumed to be marginal. So, overall, probably a net positive.

Also,  apparently no Chess or Motown yet. While they are both owned by Universal, it appears that they are not included in the deal.

NYT coverage here: https://www.nytimes.com/2011/01/10/arts/music/10masters.html

NPR coverage here: http://www.npr.org/blogs/therecord/2011/01/10/132803768/200-000-recordings-donated-to-library-of-congress (includes the statement “only 14% of pre-40’s music is commercially available today”. Which fits closely, of course, with Sturgeon’s Law)