It has been known for a while that firewire allows direct memory access (DMA) which allows you to

This also includes unlocking a locked windows machine.

I was a a DefCon group meeting and one of the guys told me there was a similar attack for unlocking a windows machine but with a USB stick. He was pretty adamant, but I don't think it is possible since I don't believe there is DMA with USB. Can someone shed some light on the subject. I am not talking about AutoRun.

The way I understand it is that USB does indeed use DMA, bypassing the CPU and thus enable decent transfer speeds.   When I did research on this, people are saying that Firewire is a BUS, while USB is a PORT.  While both use DMA, Firewire is more unrestricted.   When Microsoft implemented Firewire, they assumed that it was going to be running very fast things like hard drives.  Interestingly enough, people are saying that the DMA works on eSATA as well.   

I am not sure if any of this is true, but that's what I found while researching this topic.

I think that logically it should work, but I was never able to get it to work or find anyone else that has.   I think it's a matter of how Firewire vs USB are implemented in Windows.