You're exactly right -- if all your EBS volumes are served out of the same SAN hardware, or worse, served from the same physical spindle in the SAN, you're still toast.
When I was testing with bonnie++ though, I saw 2x better I/O numbers with raid5 or raid6 with these volumes, which I would not expect to see if they were served out of the same spindle.
SSD stuff is on the horizon. Their site is not very descriptive but SolidFire is doing some impressive stuff: http://www.solidfire.com/ - from the founder of JungleDisk
When I was testing with bonnie++ though, I saw 2x better I/O numbers with raid5 or raid6 with these volumes, which I would not expect to see if they were served out of the same spindle.
If only AWS supported SSD-backed EBS volumes, eh?