S5L8900

This is the Application Processor shared between the iPhone, iPod touch, and the iPhone 3G. Not much is known about it through official sources. According to saurik, this is an "arm1176jzf-s", if you're looking for manuals. This processor is not used in any of the newest devices, being replaced by the S5L8720, S5L8920, S5L8922, S5L8930, S5L8940, S5L8942 and S5L8945.

iBoot

 * Restore Mode - Works up to iOS 1.0.2
 * Ramdisk Hack - Works up to iOS 2.0 beta 3
 * diags - Works up to iOS 2.0 beta 5
 * iBoot Environment Variable Overflow - Works up to iOS 3.1 beta 3
 * usb_control_msg(0x21, 2) Exploit - Works up to iOS 3.1.2

Bootrom

 * Pwnage 1.0 (Ramdisk + AppleImage2NORAccess)
 * Pwnage 2.0 (DFU + Malformed Certificate)

Kernel

 * BPF STX Kernel Write Exploit - Works up to iOS 3.1.3
 * IOSurface Kernel Exploit - Works up to iOS 4.0.1
 * Packet Filter Kernel Exploit - Works up to iOS 4.1
 * HFS Legacy Volume Name Stack Buffer Overflow - Works up to iOS 4.2.1

Userland

 * Symlinks - Works up to iOS 1.1.1
 * LibTiff - Works up to iOS 1.1.1
 * Mknod - Works up to iOS 1.1.2
 * Dual Boot Exploit - Works up to iOS 2.0 beta 3
 * MobileBackup Copy Exploit - Works up to iOS 3.1.3
 * Malformed CFF Vulnerability - Works up to iOS 4.0.1

Boot Chain
VROM (S5L8900)->LLB->iBoot->Kernel->System Software

One of the iPhoneLinux goals are to replace that Boot Chain after iBoot:

VROM (S5L8900)->OpeniBoot->Linux Kernel->X Server->Window Manager

Restore Mode
The common upgrade process chain is VROM->DFU Mode->WTF->iBoot->Kernel->Ramdisk->Restore Mode.

DFU Mode
To flash an older version of the iPhone software you have to let your phone reside in DFU Mode. In iTunes you have to press the option key (Mac) or the shift key (Windows) when pressing 'Restore' to be able to manually chose an IPSW.

Boot Chain
VROM->DFU Mode