I less than one year, StackOverflow has accumulated 23,305 tags (as of this post).

Should higher-end moderators have an interface to rename, combine, and delete tags at the database level (i.e. beyond editing individual messages)?

Most changes would be fairly obvious:

• Formatting inconsistencies like "visualstudio", "visual-studio", "visual+studio" could be combined.
• Word Variations: reference, referencing, references
• Obvious user error: the tag "best practices" becomes two - "best" and "practices"
• Obsolete or tags made irrelevant: visualstudio2010beta1

There will always be the fringe cases and judgment calls. If a proposed change is that controversial, it could be discussed here (meta.SO), where appropriate.

Maintaining a completely sanitized tag cloud is difficult to impossible. I'm not even suggesting that. But there might be a place for a bit of routine maintenance and sanity checking. This is best done on an on-going basis rather than all at once.