This is funny in a funny-sad way. You can search WordPress.com for tags that are single words, but not multiple words separated by spaces:

• http://wordpress.com/tag/spire returns results.
• http://wordpress.com/tag/viro returns results.
• http://wordpress.com/tag/spire+viro doesn’t return any results.
• http://wordpress.com/tag/spire%20viro doesn’t return any results.

I’m trying to think of a valid reason why. Identifiers can’t have spaces in the WordPress.com backend system? Hello, DOS!