One of the features of Linode is supposed to be the ability to "share [a] disk image between configuration profiles". But I cannot find any documentation of how to do this. Could someone enlighten me?

You have a chunk of disk. You can divide that disk into different partitions. Then in a configuration profile you allocate what partitions are assigned to that profile when booted. So any profile can share some or all partitions.

You can only boot and have running one configuration at a time.

Thanks for the clarification.

I guess the key qualification is "between configuration profiles". I naively read that as "between Linodes".

Nah, I'm pretty sure disk is local to each server. So even if it was possible, both linodes would have to be on the same physical host and then you'd have the usual issues of having some soft of cluster-aware system mounting disks on more than one host.

You can copy images between linodes though.

NFS + Private IP make sharing data pretty easy, as long as your nodes are in the same datacenter.