Update, November 3rd: following the complete announcement on the prim changes and the timeline, full details on the changes for Mainland, and those to be applied to private regions are now available in this post.
The forums in Second Life and elsewhere were agog on Tuesday, November 1st when Mainland users started noticing something amiss following the weekly Main (SLS) channel deployment and restart.
Region Land Impact / prim allowances for Mainland regions on the SLS channel had all increased!
Was it an error? Or if not a mistake, what was going on?
People started calling support to find out more, while speculation spread from the official forums to places such as SL Universe. The speculation prompted Patch Linden to issue a short statement:
Hi! Over the next couple of days there will be some changes coming. The deploy will happen in stages. So as you see these changes rest assured it is normal and is happening as intended.
On November 3, we will post an official blog along with a video where I will have all of the information and details regarding these changes.
Keep an eye on the Official Blog, on Thursday for more info. 
So does this mean that Mainland LI / prim allowances have increased? It does.
Does it mean that other increases are liable to follow? That is something you’ll have to wait until Thursday, November 3rd, to find out. I’ll have more at that time as well, including a chat with Patch, once the official announcement has been made.
With thanks to Patch and Pete Linden.
Inara Pey: Living in a Modemworld 