Just checked nokia firmware repository with navifirm+ and found the 1750.805.8773.12140 firmware to be available for the Nokia Lumia 800 (RM-801). Currently only chinese unbranded versions (so called Country Variant) are released but I guess other versions can't be far away now.
There is no official changelog available at the time of writing, but it is highly likely this firmware:
* Brings Windows Phone 7.5 Refresh (code named TANGO) [judging by the 8773 version number]
* Brings Internet Sharing (tethering) capabilities (rumours)
* Fixes all known audio bugs
* Fixes/improves camera (overall image quality/panorama mode)
Someone cares enough to try this out? :)
* Firmware may contain local substitutes for Facebook and Twitter, avoid if you rely heavily on these services
* Filter on '800' in Navifirm+ to see RM-801 firmwares in the list
* How to flash?!? => See this video on youtube AND (recommended) read this thread to get a better understanding.
* XDA user surya467 has tried the new ROM, his findings so far:
export contacts to SIM
advanced network settings for calls and messages
additional option in video mode - the quality of
Xbox animated tiles
new sign on the top bar
another XDA user 'Sirstoner ' tested the firmware; his conclusions:
* No Facebook integration in Contacts
* No Twitter account selection possible
All the other stuff has been mentioned, tethering, settings and camera updated. But for everyone who uses facebook this one isn't any good... Damn, cuz I use facebook a lot :P
Lots of China customization, lots of Chinese social networks intergration (cool logo's) but very pointless if ur a FB using european.
Display language: UK / US / Chinese 1 and Chinese 2
Region format: World
System locale: World
Browser language / search: World
WLAN hotspot tested and confirmed working. So if that is more important for you than Facebook, there you go
Some news sites report this firmware as being Chinese lumia 800 specific. The weird thing is that this is not true. While the rom itself is targeted for China (due to the language and fb/twitter leftout), the targetted device is the 'International' version of the Lumia 800 (RM-801). There is also a chinese version of the lumia 800, this is referred to as Lumia 800C (RM-802) in the repository. Last we have the US version of the Lumia 800 (RM-819).
Both the 800C (RM-802) and 800 'International' (RM-801) have the 12140 firmware available in the repository, while the RM-819 has not. Before today only the 800C had the socalled 12120 firmware, while the 800 'International' was left at 12070. Either way, for better world-wide support (fb/twitter/more languges) we have to wait for nokia to release 12140 firmware for other continents. My guess is this will be soon.
Last, updating this ROM with previous language packs works, but only for old functionality. Obviously these language packs (of firmware <= 12072) miss translations for new created sections such as the WiFi Hotspot menu.
The c in 800c could also stand for CDMA as suggested by xda user nikatnight.