Recently the jQuery Impromptu developer Trent Richardson released new 4th version of this plugin.
New version add's advanced event handling, more about it at Trent's blog:
http://trentrichardson.com/2012/03/05/impromptu-4-0-is-more-eventful/
But there is still lack of possibility to use enter key inside prompt's form - it is blocked by default.
So I updated my extension which add this functionality and now it is compatible with jQuery Impromptu 4.x versions.
More here:
http://dobiatowski.blogspot.com/p/impromptu-extension.html