The tweet says:
Due to closing of Symbian Signed, the Certified Signed programme will not accept submissions after the 13th of December 2013. Sorry for inconvenience.
Essentially, Nokia won't be able to digitally sign applications after this date, a prerequisite for content being published in the Store. For the majority of applications, 'self-signed' applications and side loading onto user phones will still be possible, but anything low level (e.g. utilities) will likely need extra capabilities and therefore can't be signed after the new deadline.
As I've said in several stories, this is all an opportunity for developers to rethink their involvement in the Symbian ecosystem at this stage in its life-cycle. More and more developers (watch for stories over the next week) are republishing their applications as freeware in the Store, meaning that both they and their users can grab the Nokia-signed SIS files for downloading/archiving and eventual dissemination via developer web sites and application portals/side-loading. After all, certificates are expiring and applications 'disappearing' from the Nokia Store all the time, plus the Store itself may cease working at some point (possibly even before the 2016 time frame, given Nokia's impending new owners).
Yes, all very DIY, but this is how things used to be before manufacturer-run app stores arrived and, for hobbyists, the system did work.
If you're a developer, please feel free to add to this story in the comments. What implications are there for your particular titles and what are your plans for the next few months?