It would be good to allow :separator as a acts_as_taggable option. This involves:

adding to the options list
adding as a write_inheritable_attribute
adding to the class_inheritable_reader list
removing it from the option list
replacing the various calls of the form 
          options = { :separator => ' ' }.merge(options)
 with
          options = { :separator => tag_separator }.merge(options)

I've done this in my local copy; you're welcome to it, but it's easy enough to do.

Best,

Will Fitzgerald (will dot fitzgerald at pobox dot com dot fake -- take out dot fake)