Although it sounds fancy, this reminds me the 90s. Sorry.
(I'm involved in Seesmic, as both a disclosure but also a "direct to the source" opportunity!).
Of course most of the Internet users today, weren't users in the 90s, so this doesn't mean you wont find users to appreciate what you are offering.
The idea is to allow 3rd party developers to build scripts/plugins/etc that users can download and change their twitter client experience.
Think less Eudora and more Firefox plugins.