I once spent a couple of hours in a BA 747-200 simulator, many years ago and I asked the same question. Basically, all the 'dials' were ammeters or voltmeters connected to D to A converters and a computer interface. This was polled every few milliseconds and minute changes made to the output voltage or current. It was entirely up to the computer as to what was displayed and when.
With glass cockpits, its even easier, although my guess would be the screens are real, because nothing that good exists next to a PC. I remember seeing my first glass cockpit - a 757 on way to Arlanda - and being amazed at how black the 'raster' was, even in blinding sunlight.
Roger.