How do I upgrade to Snow Leopard without a CD drive?

"I just had this exact same problem. A lot of people have been suggesting to use an external hard drive to put the Snow Leopard installation media on. This can be slow (eww, USB), and not everyone has a large enough external drive.
All I did was re-size the hard drive inside my Macbook to create another, smaller partition (10GB) and put the Snow Leopard installation files on there. Here's how:
Create the partition

Open Disk Utility

Select the Hard drive you want to resize and put the installer on. This is not 'Macintosh HD', in my case it is '260.06 GB Hitachi HTS5...'

Select the 'Partition tab' and decrease the size of your main partition (In my case 'Macintosh HD') by 10GB. You can do this by dragging the partition up, or by manually changing the value in the text box.

Press the + (plus) to create a new partition. The size of this new partition should be 10GB, it should be formatted with 'Mac OS Extended (Journaled)' and the name should be something snazzy like ""Snow Leopard Installer"". All up, it should look like this:

Once done, hit 'Apply'. This should take 5-10 minutes, but don't worry if it takes more (especially with a larger hard drive). You should leave your system alone whilst it is applying the changes, and it should not take any longer than 1 hour.

Copy the installation media

Open Disk Utility, and drag the Snow Leopard .dmg installer into the pane on the left.

Select the Snow Leopard .dmg you just dragged in from the list on the left, then the 'Restore' tab

Drag the Snow Leopard .dmg from the list on the left of Disk Utility into the 'Source' field, then drag your newly created 'Snow Leopard Installer' partition into the 'Destination' field.

Make sure ""Erase Destination"" is checked. It should look like this:

Click 'Restore'. It took 15 minutes to complete on my Macbook.

Booting into the installer

Once you have a Snow Leopard Installer partition set up, restart your computer and hold the 'Option' (or alt) key at boot up and select 'Snow Leopard Installer' from the list. The Snow Leopard Installer should successfully boot up and you should be able to upgrade to Snow Leopard!"

"If you have access to an external USB or Firewire hard drive and another computer that does have a working CD drive, you could use the other computer to clone the Leopard CD onto the external drive, and install from there.

Put the disc in the drive.
Open /Applications/Utilities/DiskUtility.app
Click on the CD drive icon in DiskUtility.app
Click the [+New Image] icon.
Choose to save it to the external drive.
When it's done, eject the external drive, and plug it into your MacBook Air.
Install from there, and enjoy :)"