Hirch,
It is my understanding that the core of Starfall Cluster Sourcebook has been written. Martin received submissions from contributors in the early part of last year, including a racial write-up on Dolphins and some basic stat blocks from myself.
Unfortunately, there was a problem with the printer getting the Second Printing of the THB out the door, and by the time that issue got resolved, priorities had changed.
There's a number of documents that I've heard tale are ready or near-ready. The problem lies in those steps in the production process after writing is complete. Artwork is sorely needed, as is layout expertise. Then QLI has to line everything up with the printer. I'm sure it's also limited on available cash needed for creating the printed product, and on the throughput of the printer. And that's just the stuff I can imagine... I'm sure there's more complications I can't even conceive of with my admittedly limited knowledge of d20 publications.
I am confident that QLI has plenty of material to provide the Traveller public for several years, and there's more coming down the pipe. The problem seems to be in getting it into production, and that can be a pretty big hurdle.
Now, just to let you know, this is all speculation, and I do not share any inside information (save that I know QLI has products written and waiting their turn in the queue, which I think is an open secret, so to speak). This is just a guess and could either be dead on, or just as far off base as you can get.
Still, I hope it helps,
Flynn