Unfortunately, all the Cinema Displays are DVI-D (i.e. digital) only, so there's no way to connect them directly to an analogue output like VGA or the older Mac DA-15 connector. There are adapters available to make the connection, but they don't come cheap! For example:

http://www.cableuniverse.co.uk/catalog/cables/vga-to-dvi-d-converter.html

http://www.nti1.co.uk/vga-dvi.html

There are a lot of cheap converters available, but these won't work because they're designed to go the other way -- from a DVI computer to a VGA monitor. They take advantage of the fact that the outputs on pretty much all computers are DVI-I which means they have both analogue and digital signals on the one connector.

To be honest, you'd probably just be better buying a cheap flat panel that has a VGA input for your older Macs [)] ]'>