Nikon D3100 firmware version 1.01 reduces a magenta tint along the bottom of still images shot at high ISO sensitivity when the sensor was hot, typically after shooting in live view mode or capturing movies for a period. It also fixes several problems related to live view mode. When Subject-tracking AF was enabled for live view shooting, the focus point could sometimes slip off the intended subject, an issue that Nikon says has been resolved. Live view images shot in burst mode, meanwhile, didn't always display correctly in playback mode, another issue said to have been corrected. Finally for live view shooting, Nikon has reworded the message displayed when the sensor's temperature limit was reached, although it doesn't state what the wording change was.
Another playback-related issue is that in the previous firmware, images that were altered using the in-camera retouch functions were incorrectly shown on the date they were retouched, rather than their capture date, using the calendar view. Also fixed is a problem with a magenta color cast appearing in Raw images created with the overlay function, if they were subsequently fine tuned in Capture NX 2 or ViewNX 2.
The remaining changes relate to noise reduction, and it's not entirely clear from the release notes what has been changed. The functioning of the Noise Reduction setting as listed on page 134 of the original D3100 reference manual is restated, with the only difference that we can see being a mention of sensor temperature among the cues that can cause the NR function to kick in. It's possible that this is newly taken into account by the NR function, in addition to the cues described in the manual -- an exposure time longer than eight seconds, or a high ISO sensitivity. Finally, Nikon states that it has revised the in-camera help information relating to noise reduction, so as to clarify the new behavior.
More details can be found on the Nikon Service & Support website, along with the firmware downloads and installation instructions for both Windows and Macintosh platforms.