We are discussing it, we have a ticket filed on it here:
http://code.google.com/p/growl/issues/detail?id=536.  You can star
that and track it for progress updates.  Note that for best experience
this will likely require a framework update in the apps as well as a
Growl.app update.
As an alternative to a Growl shipped solution, there is the Bark
plugin, it is third party, we do not support it or create it, although
we are in touch with its developers.  The Bark plugin is an action
plugin, and if you want to use it as your global default, you need to
set your global visual display to use none.  This is bugged in 2.0
(fixed in 2.0.1 which is in review) and the workaround is instead of
setting it in the UI, to quit growl, and use the following command in
terminal.app:
defaults write com.Growl.GrowlHelperApp GrowlDisplayPluginName ""
then relaunch Growl.