It's certainly possible to develop on a Windows machine, in fact, my first application was exclusively developed on the old Dell Precision I had at the time :)
There are three routes;
- Install OSx86 (aka iATKOS / Kalyway) on a second partition/disk and dual boot.
- Run Mac OS X Server under VMWare (Mac OS X 10.7 (Lion) onwards, read the update below).
- Use Delphi XE4 and the macincloud service. This is a commercial toolset, but the component and lib support is growing.
The first route requires modifying (or using a pre-modified) image of Leopard that can be installed on a regular PC. This is not as hard as you would think, although your success/effort ratio will depend upon how closely the hardware in your PC matches that in Mac hardware - e.g. if you're running a Core 2 Duo on an Intel Motherboard, with an NVidia graphics card you are laughing. If you're running an AMD machine or something without SSE3 it gets a little more involved.
If you purchase (or already own) a version of Leopard then this is a gray area since the Leopard EULA states you may only run it on an "Apple Labeled" machine. As many point out if you stick an Apple sticker on your PC you're probably covered.
The second option is more costly. The EULA for the workstation version of Leopard prevents it from being run under emulation and as a result, there's no support in VMWare for this. Leopard server, however, CAN be run under emulation and can be used for desktop purposes. Leopard server and VMWare are expensive, however.
If you're interested in option 1) I would suggest starting at Insanelymac and reading the OSx86 sections.
I do think you should consider whether the time you will invest is going to be worth the money you will save though. It was for me because I enjoy tinkering with this type of stuff and I started during the early iPhone betas, months before their App Store became available.
Alternatively, you could pick up a low-spec Mac Mini from eBay. You don't need much horsepower to run the SDK and you can always sell it on later if you decide to stop development or buy a better Mac.
Update: You cannot create a Mac OS X Client virtual machine for OS X 10.6 and earlier. Apple does not allow these Client OSes to be virtualized. With Mac OS X 10.7 (Lion) onwards, Apple has changed its licensing agreement in regards to virtualization. Source: VMWare KnowledgeBase
At the risk of getting negative marks, I want to suggest that the highest ranking solution (currently the first response) violates the latest SDK Agreement as of Nov 5, 2009. Our application was just rejected for using it. Here's the response from Apple:
"For security reasons, iPhone OS restricts an application (including its preferences and data) to a unique location in the file system. This restriction is part of the security feature known as the application's "sandbox." The sandbox is a set of fine-grained controls limiting an application's access to files, preferences, network resources, hardware, and so on."
The device's phone number is not available within your application's container. You will need to revise your application to read only within your directory container and resubmit your binary to iTunes Connect in order for your application to be reconsidered for the App Store.
This was a real disappointment since we wanted to spare the user having to enter their own phone number.
Best Solution
Answering the question as asked about the size of pixels:
Pixel size on an iPhone and iPod Touch
The earlier iPhones (pre-iPhone 4) Apple iPhone Technical Specifications said : 480-by-320-pixel resolution at 163 pixels per inch(ppi). About 0.006135 inches per pixel or 0.1558282 mm per pixel.
The first three iPod touch generations stated the same 163 ppi.
The iPhone 4 specs said 960-by-640-pixel resolution at 326 ppi . So pixel width is 1 inch / 326 pixels per inch or about 0.003067 inches per pixel or 0.0779 mm per pixel. You use points, not pixels. Edit: As noted in Olaf's comment, below, pixels are actually addressable, using half-points.
The fourth generation iPod touch (Sept 2010) has specs the same as the iPhone 4, 960-by-640-pixel resolution at 326 ppi
The iPhone 4S (Oct 2011) is unchanged from the iPhone 4 in terms of resolution.
The iPhone 5 (Sept 2012) specs said 1136-by-640 pixel resolution at 326 ppi. Pixel size is unchanged. Screen diagonal is 4 inches.
The iPhone 5C and iPhone 5S (Sept 2013) have the same resolution, pixel size, and diagonal as the iPhone 5.
The iPhone 6, 4.7 inch, (Sept 2014) specs are 1334-by-750-pixel resolution, at 326 pixels per inch (ppi). pixel size is unchanged from 4, 4S, 5 5s.
The iPhone 6 Plus, 5.5 inch, (Sept 2014) specs are 1920-by-1080-pixel resolution, at 401 pixels per inch (ppi). pixel size is about 20% smaller. The pixel width is 1 inch / 401 pixels per inch or about 0.002494 inches per pixel or 0.06334 mm per pixel.
Pixel size on an iPad
Comparison of the iPad Air, and iPad Air 2, iPad Mini, iPad 2 and iPad3.
You don't need to have the iPhone SDK calculate the size of a single pixel. One option is to determine what you are running on and then select the needed mm size.
The iPhone (up to and including the 4S) / iPod Touch screen sizes, with a 3.5 inch (diagonal) display are NOT exactly 2" x 3". They are a tiny bit smaller than that. The iPhone 5 has a 4 inch (diagonal) display.
What the questioner may actually need: points. See Removers comment to the previous answer. Coordinates are specified in points, not pixels.