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