The auto-completer is not very useful for tag completion currently: it will autocomplete the whole field and does not know anything about a separator, like ",".
I've added support for this and think that /\\s*,\\s*/ would be a good default for this (that is "," surrounded by any white space).
This would be even more useful, if it could complete parts which are not at the end of the input, e.g. "f_ , bar" would complete "foo" at the cursor position "_".
For this to work however, we would need to get the current cursor position and there appears to be nothing useful in jQuery or jQuery UI core already (I've found e.g. http://plugins.jquery.com/node/6631).
This could be added later quite easily, by not only remembering the current prefix, but also a suffix (and massaging the value according to the current cursor position).