just downloaded and installed the iOS 4 SDK and as my root OS X partition is rather (too) small, I put it into a custom location /Users/Developer.Snowleopard/.
This causes the iPhone Simulator to crash and compiling gives an error like:
ibtool failed with exception: Interface Builder encountered an error communicating with the iPhone Simulator. If you choose to file a crash report or radar for this issue, please check Console.app for crash reports for "Interface Builder Cocoa Touch Tool" and include their content in your crash report.
...
dyld: Library not loaded: /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.0.sdk/System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
Referenced from: /Users/Developer.SnowLeopard/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.0.sdk/System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libLAPACK.dylib
Reason: image not found
The cure – a softlink:
$ ln -s /Users/Developer.SnowLeopard/Platforms /Developer/Platforms