Well... you can do the math yourself:
Check your iBook's adapter for how much power it draws. I seem to remember those being around 45W max, so I'll use that as an example, but if you're serious you should probably try to get your hands on a cheapie Wattmeter to check what it really draws under normal circumstances (probably significantly less than max).
Then find out how many amp-hours your car's battery holds. You may even want to replace it with a beefier one if that's feasible.
In very round numbers, say you've got a 50 amp-hour battery and the iBook is drawing 42 Watts (which works out to about 3.5A at 12V). That would mean that you could (totally theoretically) run the iBook for 25/3.5 = about 7 hours before discharging your battery halfway. Not that bad, if my assumptions about amp-hours are anywhere near accurate.
I know almost zip about car electrical system, but a really quick Google search turns up that alternators likely put out at least 50 amps, which shouldn't have any trouble handling an iBook, and headlights are usually at least in the 50W range, so an iBook shouldn't be at all taxing on a newer car with a decent alternator and battery.
