This is not a recent problem, but given I have stumbled upon it twice in the last week I thought I'd share the problem and solution with the community here, specially as the 2 cases I had were becoming critical as package distribution was not working.
So, what is the issue? Well, first packages are not synchronising between the Notification Server (6.0 SP2 R13) which is a problem - specially in the week after MS12-020 was released).
What is blocking the package delivery? Honestly I can't tell. However the common point between both cases was that the packages where delivered on the PS under the "Altiris Agent" directory, with one guid per package.
As I couldn't get the package servers to synchronised I went to a random package and removed the Package Server Download Alternate Location entry. After running a client configuration update the package server started synchronising all packages.
This was true for both customers, and is worth remembering.
Now the "Package Server Download Alternate location" issue is known and fixed, but here is a refresher on where to find the fix: