Unfortunately, not long ago Apple decided to put much brighter tubes in their iMac monitors (for flashier performance with iMovie etc) and now none of them can really be calibrated down to typically recommended luminosities. Worse still the latest machines are going for glossy screens, which are pretty awful for colour work in my experience. Bit of a bugger if the monitor is built in, really.

And Apple are by no means the only ones to be using very, very bright tubes in their monitors these days - many BenQ, Dell, etc etc models suffer from this problem as well. It's not that bright tubes per se are a problem - in fact they can be very useful - but some of these screens lack controls to bring the brightness down to sensible levels for day to day use - some of them can't bring the monitor down below 250 candelas, which is ludicrously bright for day to day computer usage.

Contrast this with any Eizo screen that can easily reach the same high luminance figures, but has excellent, precise controls for bringing the brightness down as well (in fact Eizo screens come with special software that can detect which application you are running and dynamically switch between modes so that you can instantly take advantage of both high brightness levels when needed (say when watching a movie) and lower brightness levels when needed (say photographic retouching).

Now - if you own one of these new high brightness monitors, it's not all bad. There is in fact no absolute standard for where luminosity should be anyway. And you do want your monitor to be the brightest thing in your field of vision when doing photographic retouching. What's most important is the relative relationship between your ambient light level and the monitor luminosity. (You can check this with the ambient light check feature if your screen calibrator has one).

Basically, should you own a screen that can't come down to normal figures like 120 candelas, this means you should keep your room a little lighter than is normal for this sort of work (not such a bad thing, necessarily). Ideally your room lighting level is 30 to 50% less bright than your monitor. This means your eye will calibrate to the monitor's whitepoint (as your eye always goes to the brightest white in its field of view and chromatically adapts to that whitepoint). If you get this relative relationship right, you should still find these monitors useful. That said, staring all day into 250 plus cd/m2 is very tiring on the eyes, and personally I'd look to a better screen as a possible future purchase if photographic retouching is a big part of what you do, or indeed simply if you spend a long time in front of your computer on a regular basis.

This, by the way, is why Colour Management works when your monitor has a whitepoint of 6500K and your print viewing lights are at 5000K, or less. In each case, your eye auto adapts to the whitepoint and it's how colour behaves relative to that whitepoint that is important. If you had the light levels equal, and were doing side by side matching, you'd be better off calibrating your monitor to the same white point as your lighting (i.e. around 5000K). This is not recommended as it takes you well away from the monitor's native whitepoint, and there's a big price to pay for that on an LCD - it tends to promote pretty severe stepping across the gamut.