Variable difficulty ("var diff") is now enabled for those using the getwork protocol. It was already enabled for Stratum and GBT.
Let me know if you have any issues. Some older miners may not be happy with it, but they should still be able to mine.
I am observing a decent drop in CPU usage on the server, so I am quite happy with it so far.
Max difficulty is still 16. This will be increased later as more ASICs turn up.
Is this done on a per worker basis, or per account? If i have a few slow graphics cards, then bring up an asic what happens?
This is per worker. If you have a slow GPU and an ASIC then you may want to use different worker accounts for them. It's not the end of the world if you don't though, it just adds variance.