This isn't currently listed anywhere on the EBS/EC2 documentation, and it's something that's fairly useful.  If you want to attach an EBS volume to an EC2 instance you can use any device that isn't currently in use, and that matches the following regular expression:

/dev/sd[a-z][1-6]?

That means /dev/sdc, /dev/sdd1 and /dev/sdz6 are all valid, if they aren't currently being used.  /dev/sda7 and /dev/sdb11 would not be valid devices.

I stumbled across this in the AWS Forums, posted by AndrewC@AWS.  Pay attention to his warning about available devices when you've got partitions created by fdisk.

The device names such as /dev/sdh1, etc. are a little tricky. If you attach a volume to /dev/sdh and then use fdisk to partition it, the OS will automatically create /dev/sdh1, /dev/sdh2, etc. according to the partitions you create, so then you can't also attach an EBS device to /dev/sdh1 in that case.

The tremendously useful Elasticfox extension for Firefox implies that the only valid devices are /dev/sda through /dev/sdp, hopefully this will clear up some confusion.