Many of you already know this, because you went to Lotusphere or saw the post on Ed's blog, but I'd thought I point out this site again, since I just ran across it myself through some internal communications.
I've also been reading up on the customization capabilities. Now you'll not only be able to build server-side realtime applications, but also extend those right into the ST 7.5 client experience. Since this is built on Eclipse, the client has well-defined extension points throughout.
More information is being prepared, including samples and documentation, so stay tuned.
What kinds of things would you want to add into the ST client? You've seen the screen shots. We're counting on our developer community to come up with some cool (or kewl, as Rocky says) add-ons.
Sametime 7.5 Info
IBM Workplace Application Development