This has happened to me a couple of times, and I think this would be an excellent improvement.

I have deployed an application (for bugfixes and unplanned enhancements) twice between planned releases, but the items that I have been working on that were released are still marked as being fixed in an upcoming version.

If you could allow users to specify the "fixed" version as an "Unreleased version", then then when a version is released, the system would check through the list of issues to see which ones had "Unreleased" specified as the fixed version and set it to the recently released one.

That way, if I have release version 0.8, and I'm tracking issues to be released in 0.9, and I have a few sudden critical bugs that need to be fixed and released I can slip in version 0.8.1 and release some bug fixes and have anything that was fixed to that point tracked as being fixed in 0.8.1 instead of 0.9.

From an implementation perspective:
Creating or editing an issue:
Selecting "unreleased version" as the fixed version records fixedVersionId as -1.

Release a version:
When an administrator (or lead) releases a version, loop through issues for the project, if fixedVersionId = -1 then set fixedVersionId = justReleasedVersionId.

Questions or comments: jira@pollensoft.com