A friend with an '09 IS250 has found that the navigation screen does not dim, or reverse contrast from the bright daytime view to the more subdued view in dark environments and nightime driving. My LS460 does this automatically. Is there some special setting that must be toggled for auto-dimming, or does the IS250 system not have this feature?

When the instrument panel light control knob is turned fully clockwise there's a click and in this position the nav screen always stays in the bright daytime setting. Turn the brightness control down a little and you should find the nav screen changes between bright and dim automatically.

Is is just the nav screen, or is it the instrument panel too? If it's the screen and the IP, do what Roger says. If it's just the nav screen, at night, hit the "DISP" button next to the screen, and then hit "Day Mode" on the screen (the Day Mode button will not appear during the day).