The final release of Lightroom 4.2 is now available on Adobe.com and through an update mechanism in Lightroom 4.  According to Adobe, "The goal of this release is to provide additional camera raw support, lens profile support and address bugs that were introduced in previous releases of Lightroom."

Lightroom 4.2 supports Sony RX-100, Canon T4i

Finally, the Sony RX-100 is supported! Other supported cameras such as the Nikon D600 are listed below.

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From the Lightroom 4.2 Release Notes:

Bugs Corrected in Lightroom 4.2:

The following bugs that were part the Lightroom 4 releases have been corrected. The team appreciates the very detailed feedback the community has provided on Lightroom 4 and we’re excited to correct a number of issues experienced by our customers. These issues have been FIXED:

Stacked photos were hidden in both the Grid view and Filmstrip. This occurred when photos get unstacked as a result of enabling auto-stacking.

Folder stacks with virtual copies were unstacked when moving to another folder.

When scrolling through the filmstrip (at somewhat fast rate), occasionally a “gray” image was displayed for an instant and then replaced by the correct image preview.

The Navigator Panel in Map did not pan correctly.

Parents and synonyms of “do not export” keywords also did not export..

Occasionally were unable to publish videos to Facebook.

Audio did not play in video files after creating a slideshow that contains music (Win 7 only).

Image select arrow on keyword list didn’t display when keyword list contained a large number of keywords.

Unable to edit Lightroom photos as jpegs in Photoshop Elements.

Carriage Return in either the Title or Caption field invalidated a Flickr upload.

Tether capture bar disappeared after deleting photo (Win only).

Back Cover of Book layout had Horizontal text on spine rather than Vertical.

Sharpening was not applied to books that were exported as JPEG files.

Double-byte characters were not exported to Books saved as PDF files.

Unable to type accented character in book module in LR4.

Luminance Noise slider was unresponsive when Clarity is greater than 0.

Develop keyboard shortcuts (+ and – keys) no longer worked in older process versions such as PV2010.

New Camera Support in Lightroom 4.2:

Canon EOS 650D / Rebel T4i
Canon EOS M
Fujifilm XF1
Fujifilm X-E1
Fuji FinePix F800EXR
Leaf Credo 40
Leaf Credo 60
Leica S
Leica D-LUX 6
Leica V-LUX 4
Nikon Coolpix P7700
Nikon 1 J2
Nikon D600*
Panasonic DMC-G5
Panasonic DMC-LX7
Panasonic DMC-FZ200
Pentax K-30
Samsung EX2F
Sony Alpha NEX-5R
Sony Alpha NEX-6
Sony Alpha SLT-A99V
Sony DSC-RX100

*Please note that the support for Nikon D600 is preliminary and there is a minor risk that the appearance of your images may change when the final support for Nikon D600 is available in an upcoming release.