Bootrom

The bootrom (called "SecureROM" by Apple) is the first significant code that runs on an iDevice. The bootrom is read-only. Finding exploits in the bootrom level is a big achievement since Apple won't be able to fix it without a hardware revision.

Old & New bootrom

Certain models, including the iPod touch 2G and iPhone 3GS, have different bootrom versions. These are most commonly referred to with the terms "old bootrom" and "new bootrom." These "new bootrom" devices were released after 9 September 2009 and have the 0x24000 Segment Overflow fixed. While the new bootrom revisions have an exploit, the exploit needs the assistance of a firmware-based exploit to achieve an untethered jailbreak.

You might also be looking for Apple's stage 2 bootloader, which also uses the "iBoot" name.

Usually also looking at the CPRV (Chip Revision) tag will also tell you whether the device is new unit or not also.

Finding bootrom version

From the model number (iPod touch 2G)

If the second character of your Model Number is "B" (e.g.- FB533, MB533, or PB533), your iPod has the old bootrom. If the second character is "C" (FC086, MC086 or PC086), your iPod has the new bootrom.

From the serial number (iPhone 3GS)

The third digit of the serial number identifies the year of manufacture (9=2009, 0=2010, 1=2011, 2=2012), while the fourth and the fifth indicate the week. There is a gray area between week 40 of 2009 (??940??????) and week 45 of 2009 (??945??????) where some devices have new bootrom whilst others have old bootrom. . Any iPhone made after Week 45 of 2009 (??945?????? and higher or ??0???????? serials) has the new bootrom.

From the DFU Device descriptors (all devices except S5L8900)

Windows

Connect Device & Enter DFU Mode
Open Device Manager, find USB controller, subitem Apple Mobile Device USB Driver
Right-Click & click Properties
Go to Details tab & select Device Instance Path in the dropdown box
The end of the info string will show the bootrom version

Mac OS X

Connect Device & Enter DFU Mode
Go to System Profiler, and under the Hardware category, go to USB, and click on Apple Mobile Device (DFU Mode)
The end of the Serial Number string will show the bootrom version in brackets (ie: [iBoot-574.4])

Linux

Make sure your distribution has usbutils installed. (most distributions have it by default)
Connect Device & Enter DFU Mode
In terminal, run sudo lsusb -v
Find the line that says iSerial and your bootrom version will be at the end of the line.