Something iffy has definitely happened to SCSI. I have a CD writer & a slide scanner connected to an Adaptec 2906 SCSI card, and neither of them shows up in System Profiler under 10.3. I spent ages reinstalling everything I could think of, but nothing made any difference.
Eventually, I just tried using them; both work fine! The scanner works with VueScan, and the CD writer works in the finder and in Retrospect, exactly as in 10.2. Great relief, but still worth reporting to Apple.
Barney.