The issue that caused the current fork has been addressed on the develop branch
and a new release to master will be done by the end of the day courtesy of /u/jeffehhh and the rest of the Go team's hard work over the weekend. Once it's out, we'll post it everywhere so everyone can update and get the testnet back on track
The issue was essentially caused by a change in the latest client which was meant to be an optimisation and make things faster, but ended up making it so nonces were not being checked for validity. This meant one could make blocks with arbitrary nonces (e.g., no work had to be provided).
We've worked quickly to address this issue and apologise for the inconvenience.
Testnet's gunna test. Thanks again for your patience!
One last thing (as Vitalik said in a previous post):
"Please do continue whacking Olympic and looking for bugs. We highly appreciate it
That said, perhaps stop with the "10kb dummy data" spam and move on to something else, like creating a contract that consumes 1m gas worth of EVM computation and sending transactions to it. We'd like to make sure all bases are covered."
Update 9/6/15 12:13 BST - Network is looking to be pretty much back on track and it would seem we have recovered from the fork.