With any luck, this will give an error message if the user tries to install on OS X 10.1.x. I have no idea if the installer will even open and no way to test it. Worst case, installer still works for >= 10.2 and crashes for < 10.2. The same as before. Best case, it gives a message about not being compatible and then quits.

stepmania/PBProject/Installer/main.c

@@ -8,8 +8,40 @@
  */
 
 #import <Cocoa/Cocoa.h>
+#import <Carbon/Carbon.h>
+
+enum
+{
+    kMacOSX_10_2 = 0x1020,
+};
+
 
 int main(int argc, const char *argv[])
 {
+    long response;
+    OSErr err = Gestalt(gestaltSystemVersion, &response);
+    CFStringRef error = NULL;
+    
+    if (err != noErr)
+        error = CFStringCreateWithCString(NULL, "Couldn't determine OS version.", kCFStringEncodingASCII);
+    else if (response < kMacOSX_10_2)
+        error = CFStringCreateWithCString(NULL, "StepMania will not run on any version of Mac OS X 10.1.x "
+                                          "or earlier.", kCFStringEncodingASCII);
+    if (error)
+    {
+        CFStringRef OK = CFSTR("OK");
+        struct AlertStdCFStringAlertParamRec params = {kStdCFStringAlertVersionOne, TRUE, FALSE, OK, NULL, NULL,
+            kAlertStdAlertOKButton, NULL, kWindowAlertPositionParentWindowScreen, NULL};
+        DialogRef dialog;
+        SInt16 result;
+
+        CreateStandardAlert(kAlertStopAlert, error, NULL, &params, &dialog);
+        AutoSizeDialog(dialog);
+        RunStandardAlert(dialog, NULL, &result);
+        CFRelease(error);
+        
+        exit(0);
+    }
+    
     return NSApplicationMain(argc, argv);
 }