I have Snow Leopard 10.6.8 and am running XCode 3.2.6. Attempting to compile SVN fails for me as well out of the box. This isn't quite the same environment, but in case it helps, this is how I fixed it.
Download the Justice Edition and examine the source directory, in particular src/xcode/Frameworks. Note the file src/xcode/Frameworks/version.txt. Read it. ;) To summarize, SVN has an out-of-date set of SDL frameworks.
To fix, simply replace src/xcode/Frameworks in SVN with src/xcode/Frameworks from Justice Edition. I didn't need to make any other changes.
Now, Lion and Mountain Lion eliminated PowerPC compatibility (eg. Rosetta). I suspect some of the Sauerbraten xcode project settings that maintain compatibility with PowerPC are also getting in the way of your compile.
So, open the info panel for the project by right-clicking on the top sauerbraten folder in the Groups & Files panel and select Get Info. In the resulting Info panel select the Build tab.
In the build tab you'll see a long list of settings. Note for example Valid Architectures. Try eliminating ppc, ppc64, ppc7400, and ppc970 from that list. Also, you could try fiddling with the Base SDK setting. I know things compile for Mac OS X 10.6. If it's set to 10.7, try setting it back to 10.6.
Lastly, the src/xcode/English.lproj/MainMenu.nib is a compiled nib. This is a problem. The maintainer needs to replace this with an uncompiled nib. Otherwise it cannot be edited. A workaround is to again replace it with the equivalent in Justice Edition.
