Got an email from a RAID vendor, 10.5.6 apparently is having some issues:

We have also seen on 2007 Mac Pros where the PCI-express configuration (Expansion Slot Utility) setting does not ‘stick’, it reverts to x8, x1, x1, x16 speeds always. So any PCI-express cards installed are affected also.

…so watch out. Dunno if it affects non-RAID setups, but is such a pain to roll back, if you have PCIe cards (RAID cards, HD-SDI I/O cards, etc.) probably best to wait. I’ve updated my MacBook and had no difficulties to date (last 24 hours).

Thanks to Dulce Systems for the heads up.