A very interesting discussion is taking place in the Haiku mailing list right now. A developer has created a working prototype implementation of the BeOS API layer on top of the Linux kernel, and he is wondering if the project is worth pursuing. He's got the App, Interface and Networking Kits in good shape within a few months' work.
While there are some minor downsides to having the kits on top of Linux (or one of the BSDs), the upsides include all the drivers in the world (well, the gpu driver situation could be a tad better), a rock solid kernel that works on all kinds of devices (who says BeOS can't run on a phone, mine can), and a working BeOS clone with comparatively little effort (as a musical engineer, my biggest worry was sound system latencies, but it turns out many Linux schedulers can easily be tuned to handle the loads I expect in a BeOS system.)
I think the Haiku project made a monumental mistake in not using an existing kernel - it's simply no longer practical for a small project to keep up with the hardware evolution, handling security requirements and so on. Sad but true, and it was sad but true back in 2001.
A very interesting and in-depth discussion follows. Both 'sides' make a lot of compelling arguments, and it gives a lot of insight into decisions that went into the Haiku project, both past and present. For once, I have no clear opinion on this matter; both sides have merit, and it in the end comes down to what the actual developers want to work on (hint: it's not Linux).
Still, the developer in question will be putting up a repository of his Linux work, so we'll get to see what it's like.