I am also getting the exact same message with the version of the SDK that I downloaded yesterday. Here's what I get in the console. I verified that /Developer/Platforms/iPhoneOS.platform/Developer/Library/Xcode/Plug-ins/iPhoneOS Build System Support.xcplugin exists, so I'm not sure what the problem is.
There were about 20 copies of the first message, I only pasted in one.
11/14/10 9:12:14 AM Xcode[8973] WARNING: Failed to load plugin at: /Developer/Platforms/iPhoneOS.platform/Developer/Library/Xcode/Plug-ins/iPhoneOS Build System Support.xcplugin, skipping. Could not load bundle.
11/14/10 9:12:14 AM Xcode[8973] Platform validation failed for Class 'XCiPhoneOSCodeSignContext'
11/14/10 9:12:14 AM Xcode[8973] [MT] File: /SourceCache/DevToolsBase/DevToolsBase-1705/pbxcore/Target.subproj/PBXTarget.m
Line: 1603
Object:
Method: createPropertyExpansionContextWithBuildState:
Platform failed to completely load. Examine Console Log for error messages.
Backtrace:
0 0x000000010099fe19 -[PBXAssertionHandler handleFailureInMethod:object:fileName:lineNumber:messageFormat:arguments:] (in DevToolsInterface)
1 0x00000001002dab1f _XCAssertionFailureHandler (in DevToolsCore)
2 0x00000001000e9ca1 -[PBXTarget createPropertyExpansionContextWithBuildState:] (in DevToolsCore)
3 0x00000001000e9212 -[PBXTarget createPropertyExpansionContextWithBuildAction:configurationName:] (in DevToolsCore)
4 0x00000001000e8ca2 -[PBXTarget cachedPropertyExpansionContextForConfigurationNamed:] (in DevToolsCore)
5 0x00000001000e8b33 -[PBXTarget expandedValueForString:forConfigurationNamed:] (in DevToolsCore)
6 0x00000001000f8c79 -[PBXProject _validArchsMayHaveChanged:] (in DevToolsCore)
7 0x00000001000f69a0 -[PBXProject _unarchiverDidFinishUnarchiving:] (in DevToolsCore)
8 0x00007fff819f8a66 _nsnote_callback (in Foundation)
9 0x00007fff87d7e000 __CFXNotificationPost (in CoreFoundation)
10 0x00007fff87d6a578 _CFXNotificationPostNotification (in CoreFoundation)
11 0x00007fff819ef9ce -[NSNotificationCenter postNotificationName:object:userInfo:] (in Foundation)
12 0x00000001000d2cbb -[PBXPListUnarchiver decodeRootObject] (in DevToolsCore)
13 0x00000001000d0da4 +[PBXProject projectWithFile:errorHandler:readOnly:] (in DevToolsCore)
14 0x00000001000d0003 +[PBXProject projectWithFile:errorHandler:] (in DevToolsCore)
15 0x0000000100963460 -[PBXProjectTemplateClonerWizard _instantiateASCIIMacroTemplateWithContext:] (in DevToolsInterface)
16 0x00000001009634f7 -[PBXProjectTemplateClonerWizard _instantiateTemplateWithContext:] (in DevToolsInterface)
17 0x00000001009d29d6 -[PBXFileCopyingWizard _finishCopyingTemplate:filesToPreserve:] (in DevToolsInterface)
18 0x00000001009d2895 -[PBXFileCopyingWizard _checkForOverwritingFiles:] (in DevToolsInterface)
19 0x00000001009400f1 -[PBXProjectWizard finish] (in DevToolsInterface)
20 0x000000010093be24 -[PBXProjectWizardChooserWizard sheetDidEndWithReturnCode:fileSystemLocations:contextInfo:] (in DevToolsInterface)
21 0x00000001008e3fda +[PBXOpenSavePanelHelper sheetDidEnd:returnCode:contextInfo:] (in DevToolsInterface)
22 0x00007fff86bd9946 -[NSSavePanel _didEndSheet:returnCode:contextInfo:] (in AppKit)
23 0x00007fff8697fbd1 -[NSApplication endSheet:returnCode:] (in AppKit)
24 0x00007fff86891e9a -[NSApplication sendAction:to:from:] (in AppKit)
25 0x00007fff86891df9 -[NSControl sendAction:to:] (in AppKit)
26 0x00007fff8691d76b -[NSCell trackMouse:inRect:ofView:untilMouseUp:] (in AppKit)
27 0x00007fff8694e2aa -[NSButtonCell trackMouse:inRect:ofView:untilMouseUp:] (in AppKit)
28 0x00007fff8691c215 -[NSControl mouseDown:] (in AppKit)
29 0x00007fff8683634f -[NSWindow sendEvent:] (in AppKit)
30 0x00007fff8676ba86 -[NSApplication sendEvent:] (in AppKit)
31 0x00000001007fc21c -[PBXExtendedApplication sendEvent:] (in DevToolsInterface)
32 0x00007fff867024da -[NSApplication run] (in AppKit)
33 0x00007fff866fb1a8 NSApplicationMain (in AppKit)
34 0x00000001000017b4