Q:
How do I get the airbrush in phtohsop 7 to behave like it did in Photoshop 6?

A:
Select your brush shape. Under the dynamics menu in the brush palette uncheck "Shape Dynamics."

If you have a pressure sensative tablet, click the "Other Dynamics" checkbox then click directly on the "Other Dynamics" text in the palette. Set the Opacity drop down menu to "Pressure" and leave the jitter slider at Ø (all the way to the left). Leave the flow option off.

Then click the Airbrush option in the options bar (or hit option-shift-p on the keyboard).

When changing brush tips either use the shortcuts to change the size ( [ and ] ) or select your shapes from the "Brush Tip Shape" menu in the brush palette. Do not use the control-click shortcut to access the brush palette. That shortcut uses the presets which will change your dynamic settings.