The WebKit engine (the thing that powers Safari) has passed ACID 1, ACID 2 and ACID 3 tests* - They are the first team to release a publicly available engine that does so.
Now of course the real question -- When will Apple release a new Safari based on the new WebKit?
I want to see Safari 3.5 (or 4.0) within the next 3-4 months with a full ACID 3 (rendering) pass (and possibly a mobile version on the iPhone? That doesn't do very well currently). There is a lot of prestige and clout to be claimed by having the first officially released browser to pass all 3 ACID tests - It may even help make Safari the "Gold Standard" for web development, something that may help further drive Mac platform adoption.
(Author's Note -- I'm still pissed about Safari 3.1, which broke ACID 2 compliance. I refuse to install that browser as it represents a significant regression from previous Safari releases - Safari holding the title of first browser to pass ACID 2. Get it together Apple QA!)
* ASTERISK! ASTERISK! It's a rendering pass, but maybe not a full pass by the spirit of the test.
See ACID 3 requires the animation that gets you to all the colored boxes to be "smooth", and at least on my MacBook Pro (2.16GHz C2D with 2GB RAM - OS X 10.5.2) it's a little jerky still. Performance fixes that the Webkit folks should be addressing hopefully...