I've come across a bunch of guides on how to do dual booting, but I think most of them are quite outdated or complicated.
This method uses existing ones. The methods I used install the cleanest Mac OS X installation ever (a regular 10.5.6 DVD /USB image) and a clean Ubuntu installation on a GUID partitioned disk.
I followed gizmodo's guide to install Mac OS X on my Dell mini and run DellEFI to fix up things.
I partitioned my SSD
into two partitions according to my needs directly from the Mac OS X Installation's Disk Utility using the GUID partition table.
I then installed Linux the regular way, paying attention in installing Linux on the remaining space (I installed Ubuntu. GParted recognizes the GUID partitions correctly).
You can use a DVD/CD burned with Ubuntu or install it from a USB stick, it's up to you, just follow the normal guides to install Ubuntu (I'm running 9.04).
The important step here is to install GRUB onto the Linux partition rather than onto the disk boot sector (to do that, click on Advanced in the last step before starting the real Linux installation and tell it to install GRUB on the partition you've used for Linux)
After that I had to reboot Mac OS X from the USB drive and re-run DellEFI custom installation (reinstalling the booloader).
Done. When rebooting it'll boot automatically in Mac OS X, if you want to boot in Linux, simply press ESC at the boot and choose the Windows FAT32 boot option. This will start GRUB and then Linux.
I think you can install GRUB on the main boot record and then tell GRUB to start Mac OS X, but haven't tried it.
Hope it helps.
Sorry if I haven't posted links. I'll try to clean this guide for newcomers asap.