I just joined Jaiku which, I have to say, beats Twitter with its eyes closed.
Apart from it having a very sweet AJAX interface, it has some nice features which help us SEO-ers. The principle one of which is the ability to add multiple feeds to your Jaiku presence.
Any feeds that you add (which could be feeds from your blog, a Google blogsearch or whatever) show up as entries in your presence with links to the original page...with the title as the link text...all without 'rel=nofollow'. Thanks Jaiku guys, you rock!
So you can build posts with keyword rich titles, pull them into a Jaiku feed and you've got automatic inbound links to your site. Then all you need to do is put your Jaiku presence on your site and once again you've got a nice 3-way link relationship.