Obviously, your situation may be different than mine - because this may be Mac PPC specific on OS 10.5.8. However, here is what worked for me.
You can leave Dreamweaver running to test this.
Locate this file: WAFB_ServerVal.htm
The path should be YourUserName/Library/Aplication Support/Adobe/Dreamweaver CSx/en_US/Configuration/commands/WAFB_ServerVal.htm
In code view, invert lines 147 and 150 so they appear like this:
return WAValToolkit_Bundle.getValidationScript(ServerModel) ;
return WAValToolkit_Carbon.getValidationScript(ServerModel) ;
Create new PHP page and save.
Insert a CSS Form with Captcha content and test.
Hope this works for you!