On cold start or warm start the bootloader is a small program that start up the main operating system.
The Openmoko phones can use U-Boot (Openmoko default), U-boot-gena2x (alternative U-Boot) or Qi as bootloaders. Qi and U-boot-gena2x is actively developed (2012), while the Openmoko version of U-Boot is not.
Neo FreeRunner GTA02
The Neo FreeRunner has two bootloaders.
The primary NAND resident bootloader, which gets used during normal power-up, can be modified and replaced by e.g. Qi without any risk. The default NAND U-Boot loader has a size limit of 2 MB for the kernel, but you could rather easily increase it by modifying the U-Boot environment.
The (secondary) NOR Flash bootloader is a version of U-Boot which cannot be modified except by using a Debug_board. Therefore it is always available as a failsafe fallback. This bootloader can be booted into by pressing and holding the AUX button and then, while still holding AUX, shortly pressing the Power button. Note that the NOR bootloader has a size limit of 2 MB for the kernel.