My Nokia Lumia 800 got prompted for this update earlier today. FYI I am based in Gloucestershire in the UK and currently using 3 as my ISP.
It looks to be a precursor to the update below:
Messaging. Adds new messaging capabilities to your phone. You can now attach multiple pictures, videos, audio notes, and ringtones to text messages.
SIM card. Adds features related to SIM card contacts. You can now export phone contacts to a SIM card and selectively import SIM card contacts to your phone.
Other quality improvements. Includes many other improvements to Windows Phone.
This update is not available in all markets or for all phones.
The above update immediately followed: