Update: I forgot to mention that I got help from the great folks at NSCoder Nights in San Francisco when mucking about the installer and Xcode files. Particularly helpful were Bruce Spath and Dan Grover. Thanks again, guys!

Here’s how you can install the iPhone SDK for 2.2.1 on a Mac running at least MOX 10.5.5:

1. Ensure you have at least 6 gigs of disk space available. If you have tried to install the iPhone SDK on the target volume before, it may state an upgrade will be performed instead of an install. Sadly, the only solution I currently have for this situation is to uninstall Xcode using /Library/Developer/3.1/uninstall-devtools.
2. Download the SDK disk image.
3. Mount the image by double-clicking it.
4. Copy the mounted volume to a hard drive.
5. Navigate to iPhone SDK.mpkg/Contents/iPhoneSDK.dist in the copied folder and replace line 340 which should be
start_selected = "isIntel() && hasRightOS() && agreedToSLA()"

with
start_selected = "true"

6. Run the installer, selecting either the default location /Developer or another directory name if you’re looking to preserve your current Xcode installation.
7. After a successful installation, navigate from the installation directory (default of /Developer) to /Platforms/iPhoneSimulator.platform/Developer/Library/Xcode/Specifications/iPhone Simulator Architectures.xcspec, and make the following two changes.

First, replace

{ Type = Architecture;
Identifier = Standard;
Name = "Standard (iPhone Simulator: i386)";
Description = "32-bit iPhone Simulator architectures";
ListInEnum = YES;
SortNumber = 1;
RealArchitectures = ( i386 );
ArchitectureSetting = "ARCHS_STANDARD_32_BIT";
},


with

{
Type = Architecture;
Identifier = Standard;
Name = "Standard (iPhone Simulator: i386)";
Description = "32-bit iPhone Simulator architectures";
ListInEnum = YES;
SortNumber = 1;
RealArchitectures = (
i386,
);
ArchitectureSetting = "ARCHS_OLD_STANDARD_32_BIT";
},
{
Type = Architecture;
Identifier = Standard;
Name = "Standard (iPhone Simulator: ppc)";
Description = "32-bit iPhone Simulator architectures";
ListInEnum = YES;
SortNumber = 1;
RealArchitectures = (
ppc,
);
ArchitectureSetting = "ARCHS_STANDARD_32_BIT";
},


then, replace

{ Type = Architecture;
Identifier = i386;
Name = "Intel";
Description = "32-bit Intel";
PerArchBuildSettingName = "Intel";
ByteOrder = little;
ListInEnum = NO;
SortNumber = 105;
},


with

{
Type = Architecture;
Identifier = i386;
Name = Intel;
Description = "32-bit Intel";
"PerArchBuildSettingName" = Intel;
ByteOrder = little;
ListInEnum = NO;
SortNumber = 105;
},
{
Type = Architecture;
Identifier = ppc;
Name = "Minimal (32-bit PowerPC only)";
Description = "32-bit PowerPC ";
"PerArchBuildSettingName" = PowerPC;
ByteOrder = big;
ListInEnum = No;
SortNumber = 201;
},
{
Type = Architecture;
Identifier = ppc7400;
Name = "PowerPC G4";
Description = "32-bit PowerPC for G4 processor";
ByteOrder = big;
ListInEnum = NO;
SortNumber = 202;
},
{
Type = Architecture;
Identifier = ppc970;
Name = "PowerPC G5 32-bit";
Description = "32-bit PowerPC for G5 processor";
ByteOrder = big;
ListInEnum = NO;
SortNumber = 203;
},


Now go ahead and start Xcode and when you select the “File > New Project…” menu item, you should see a darling iPhone category for projects. Also, run the iPhone simulator in /(Xcode install path)/Platforms/iPhoneSimulator.platform/Developer/Applications/iPhone\\ Simulator.app, it’s really mind-blowing to run it on your desktop, especially one Apple tells you isn’t able to run their iPhone SDK.

By the way, if you don’t feel like going through these steps yourself I’ve put together iPhoneSDKonqueror, an app which will handle these steps for you in a mostly automated manner. You should buy it. It’s only USD$5 and if you appreciate the ability to use your PowerPC Mac to write apps for the iPhone, it’s the right gesture to make to me.