If it's an Apple monitor from the past 15 years it should work, but it may need an adapter.
Back when most people still used DB-9 connectors for EGA and CGA, Apple used DB-15. It was basiclly a VGA/SVGA signal, but with the pins arranged in two rows, not in 3 rows like on a modern VGA connector. You can buy adapters to convert from DB-15 pin arrangement to HD15 (VGA) pin arrangement. Many Apple monitors even came with these adapters.
More recently, Apple's first few batches of LCD monitors used a funky proprietary* connector called "ADC" Apple Display Connector. It was one cable that carried DVI, USB, and Power. Basiclly your monitor plugged directly into your graphics card with one cable.... it got DVI, plus USb and Power straight from the graphics card! You can buy adapters that plug into ADC and split it into seperate power, USB, and DVI cables.
Apple's current monitors, the ones with the aluminum frames, use a normal DVI connector. They have one cable coming off the monitor, but at the end it branches into seperate DVI, USB, Firewire, an Power cables. If you buy the 30" monitor you will need a graphics card that ouputs Dual Link DVI (sometimes called DDL) because the resolution is higher than what standard DVI can output. The DDL DVI connector itself still looks the same though.
*ADC was actually based on a draft VESA spec, but it never became official or widespread.
