The proximity sensor is used to deactivate the screen when you hold your phone to your ear during a call. The improved sensitivity should prevent any unwanted touch events from occuring (i.e. potentially "hanging-up" a call with your ear).
The Lumia 610 bug alarm tone fix means alarms will no longer sound at full volume when on a call, which should mean no more Lumia induced ear-ache. The Lumia 900 screen colours fix should means on-screen colours will be more accurate at low light levels.
As with other software update, the 8779 update for the Lumia 610 and Lumia 900 may not be universally available. This is due to the requirement for operator testing and certification.
About screen before and after update on Lumia 900
Nokia Lumia 610 (Nokia updates):
Version numbers:
- OS version: 7.10.8773.98
- Firmware version 1066.0000.8779.12201
Nokia updates:
- Enhanced sensitivity for proximity sensor performance
- Improved sound level for alarm tone during voice calls.
Nokia Lumia 900 (Nokia updates):
Version numbers:
- OS version: 7.10.8773.98
- Firmware version: 2175.2101.8779.12201
Nokia updates:
- Enhanced sensitivity for proximity sensor performance
- Improvement to your phone’s screen colours in low light conditions