I'd like to be able to run a deployment as a "dry run" to see what changes would actually be made.

Some ideas about how this might work in practice:

• Deployment tasks have an "include in dry run" checkbox. If it's not checked, the task won't run at all in a dry run
• Deployment tasks have a "Dry Run" variable and an "Commit" variable value. At runtime, the appropriate value fills ${bamboo.dryrun} so it can be passed to the appropriate script
• Create a new button "Start dry run" on deploy/selectVersionForExecute.action
• Running a deployment in "Dry Run" does not update the "current" release of an environment

Obviously it will be up to the developer to integrate the flag correctly with the tools.