In EC2 EBS, when I attach a device via UI or api, I specify the device name (eg /dev/sdf through sdp). That drive letter is consistent between reboots ? It seems implied that it is (otherwise why would they let me set it).
If you actually mean you are just rebooting the server then yes, the device name will remain. This is because you do the attach to the instance, and during a reboot everything about the instance stays the same. (Even data on the non-persistent instance disk.)
If on the other hand what you mean by "reboot" is you terminate the instance, and restart the AMI as a new instance, then technically the answer is no - but you can just re-attach the EBS drive to the instance, and use the same device name, so the effect is the same.