I have a .dvdmedia file that I got from RipIt on Mac OS 10.6.
How can I burn it back into a DVD or at least into an ISO file?
A .dvdmedia file is just a package that contains a VIDEO_TS folder. So, right click on it in Finder, choose "Show Package Contents" and you should be able to see the VIDEO_TS folder, which you should be able to burn using your software of choice.
Btw, you can also get RipIt to create a raw VIDEO_TS folder - it's in the settings somewhere. (I don't have my laptop with me right now so I can't check exactly where.)
Ripit basically takes the DVD 'as is' and doesn't strip off any structural faults that may have been deliberately put in to stop you burning it. You need an app smart enough to decode this.
The recommended App on Ripit's site is DVD2OneX2, I find Ripit's output doesn't work in Toast or Burn or in the finder. This is commercial software (40 euros). DVDRemaster is another possibility - I don't know of any free software which reliably takes you soup to nuts from ripping to burning on the Mac.