I'm beginning to implement tags within the question bank. I don't know if the implementation will be accepted for integration, but I'll at least make a patch available. I'd like to hear any suggestions or comments on how you'd like to see it done.
Here's what I have in mind at the moment as far as the UI. This is the existing question bank UI:

We'd add a field to find questions by tags:

Hopefully we could do autocomplete, similar to the autocomplete the Moodle Tracker has:

As a second question, I'd love to do this as a plugin or something so people who want it right away can have it right away. Based on a quick look at the code, it appears unlikely that there's a way to do that. If anyone who is more familiar with the question bank than I am has any thoughts on that, I'd love to hear them.