Just timed (average of two!) on my Macbook with 10.5.1:
To login screen from chime: 22 seconds
From login to ability to run an app: 7 seconds
Startup on OS X is similar to windows, there are system daemons that run before login, and user daemons that run after login. After login, I have 6 processes (outbound firewall UI, Font manager activating some 40 fonts, iCal scheduler, alarm clock, mouse gestures app and Quicksilver) that run. The first mouseover of the dock is a bit jerky for about a second, but it is useable. Annoyingly though, Quicksilver reindexes its catalog on start so it takes an additional 6-10 seconds before I can use it.
Also important for a laptop is *shutdown* speed, and that is fast, about 6-10 seconds.
The thing I love about OS X is there doesn't appear to be any slowdown over time. On my XP partition on the same machine, XP used to boot fast, but over time it takes longer to boot. This is after cleaning consistently, managing startup items with autoruns, defragging etc. And shutdown with XP is infuriatingly inconsistent, sometimes fast but often it just sits there for way too long...