I installed Chosen on "Drupal Wiki (drupal_wiki-7.x-1.2) / 7.19 core" -- The [X] delete character was missing from the items in the field, and was replaced with a hidden up-arrow (triangle).
When the "Aggregate and compress CSS files" option under Performance caching is turned off, the [X]'s are present, and all is fine. This issue is reproducible, although I could not replicate it on a Drupal 7.18 installation at Godaddy because, for some reason, that installation did not have the "Bandwidth Optimization" options.
The Chosen module provides an excellent enhancement to my project ... just what I was looking for! I hope you can fix this small problem soon, because I need to use the css performance option.
Feel free to contact me if you need further information.