Recently, I bought a 30" Dell U3011 monitor. I have a Macbook Pro unibody. From friends I have talked to, it is my understanding that Dell's 27" U711 monitor suffers from the same problem.
In order to connect to external displays, Apple has started offering Mini Displayport ports on their laptops. This is fine until you need to achieve resolutions greater than 1920x1200. Resolutions less than or equal to this can be achieved with the standard DVI to DisplayPort Mini adapter without a problem.
The obvious but incorrect solution I attempted was to use Apple's DisplayPort to Dual-Link DVI Adapter. More than a simple cable, this device has conversion hardware built in. Unfortunately, it works very sporadically, causing the monitor to sleep or to display snow. This is something that Dell and Apple do not acknowledge beyond stating that their devices are not tested against each other. The reviews of the product on the Apple site are quite damning, possibly for this reason if not others.
I tried this on my 9400 and 9600 NVidia chipsets and even went as far as trying it on an ATI Radeon chipset as well. The inconsistent snow patterns persisted and I returned the device.
Apple has released 3 firmware updates for this turkey of an adapter and none of them helped in my situation. Maybe it works for Apple Cinema Displays, but it is not what you are looking for if you own a Dell monitor.
Bottom line: Do not buy the Mini DisplayPort to Dual-Link DVI Adapter!
Next, I tried HDMI to DisplayPort Mini. Apple stores sell an HDMI cable from a third party. Unfortunately, the HDMI version that this cable is spec'd out for only goes to 1920x1200. Better to save your HDMI port on your monitor for a device that does not have the greater potential to reach the full res such as an Xbox 360.
Finally, the solution. I ordered a DisplayPort Mini to DisplayPort cable from Monoprice. Link
DisplayPort is capable of transmitting the full 2560x1600 resolution of the monitor. No chipset or conversion routines necessary, no need to buy a $100+ adapter and no need to eat up a USB port to power a chipset that doesn't work: this cable just works out of the box and works as expected.