We'd also like to know when agents go offline, but perhaps unlike most that have commented on this issue so far, we actually don't want Bamboo sending the notification.
We already have far too many point-sources of errors and events. What we really want is to tie this into our existing centralized notification and escalation mechanisms. To that end, what we would want is Bamboo to provide REST endpoints for agent status. With that, we could write Nagios checks that implements the specific checks and notifications that make sense for our organization. Likewise, others could do what they need to tie it into their existing monitoring infrastructure and policies.
Released version 0.3 of plugin with REST endpoint for agent status fetching. It's possible to use external systems to check if Bamboo agent went offline.