I just noticed that GeneTests, a voluntary listing of US and international laboratories offering in-house genetic tests, has released an updated version of their graph of commercially available tests:
You can see immediately that as the number of tested diseases continues to climb, the number of testing laboratories has plateaued - at least partly the result of more efficient testing methods, and of specialised "boutique" testing labs being swallowed up or out-competed by larger players.
As of today, GeneTests lists 606 laboratories testing for 1,705 diseases; 1,419 of the tests are "clinical" and the remaining 286 are restricted to research applications.
