Looks like a bunch of people are trying out Jaiku after "tasting" co-presence with Twitter. To me, Jaiku, which existed before Twitter, is a bunch of Helsinki mobile jocks getting into the Web 2.0 of it all whereas Twitter is the Web 2.0 crowd "getting" co-presence.
Twitter was funny for me because it was like the whole "laptop crowd" getting the "aha" that Europe and Asia had with SMS awhile back - the idea that the Internet isn't about "cyberspace" that turns on when you open your laptop, but that the Internet was something that you could carry around with you and that could ping you when it needed you.
Jaiku was neat because it was everything we had all been telling Nokia that we wanted in mobile devices, but that Nokia never seemed to deliver for us. It took a small group of mobile geeks who also got the Web to build an integrated experience.
I've been helping the Jaiku guys out a bit as an advisor and I'm also a friend of Ev's. Interesting to see this convergence of from two completely different worlds of mine (mobile and web) with the attention of the "Twittersphere" tuned in. FWIW, I don't think that either is a knockoff of the other, but I do think that they are now influencing each other. It reminds me a bit of blog software in the early days...
Having said that, there are a number of differences despite the similarities on the surface. Jaiku comes from a "presence" background allowing bluetooth proximity, phone idle time, ringer mode and other things to trigger state changes - the messaging came later. Twitter, on the other hand, is primarily messaging, which as we all know, is just a flexible and manual vector for presence information.