Activision have announced via Facebook that they will be ending support for new purchases on Guitar Hero and DJ Hero DLC.
As of March 31 only previously purchased DLC for both series will be available. After that point no new content can be bought. Rather generously, Activision has put selected Track Packs on sale for PS3, Xbox 360 and Wii, so buy up every song you want while you still can.
The servers will still be online past March 31, so you will still be able to play online and download the applicable patches should you need to.
This seems to be the final moments for the rhythm genre. Worked into the ground within a few short years, no new Rock band, Guitar Hero or DJ Hero games have been releases since 2010. With the DLC programs wrapping up, Activision are now turning off the lights and locking the doors until the eventual return.