iPhone 3.0 Beta 4 required iTunes 8.2 Pre-Release to install, and it looks like iTunes 8.2 Pre-Release requires... Mac OS Leopard 10.5.7 in order to sync notes now. Which, of course, is not yet available.
Previous iPhone 3.0 Betas have synced perfectly well under the current Mac OS release, 10.5.6.
A sign of 10.5.7's immanent arrival? Perhaps. A more interesting question is what's changed in note sync to require it where previous betas did not?
(via daveizzle on Twitter)