With new version of WP this summer (3.6 oscar), now new version of xili-tidy-tags plugin:
As example, the source now contains a new template tag – xili_tidy_tags_dropdown – to display a tag cloud as a dropdown list as expected by some users.
The example code is at (around) line 1009 in file of the plugin. The syntax is very easy like another plugin tag for cloud with a param (id_dropdown) to set the unique ID to a unique value for jQuery.
Soon more explanations…