When the iPhone 3GS was initially released, Apple did not have enough time to fix the 0x24000 Segment Overflow in the S5L8920. However, in order to flash an exploited LLB and jailbreak the iPhone 3GS, one of the following needed to be done:
- Find a new iBoot exploit every time a new firmware is out.
- Find a way to bypass the ECID checks.
- Use a bootrom exploit that allows unsigned code execution via USB.
Newer devices (iPhone 3GS with new bootrom, iPod touch (3rd generation), and subsequent devices) are no longer vulnerable to the 0x24000 Segment Overflow exploit. Many of them are susceptible to geohot's bootrom exploit (originally found in limera1n), which allows unsigned code execution over USB. Even newer devices, starting with the iPad 2, have no bootrom exploits to run unsigned code at all.
Apple added a new tag to the IMG3 File Format called ECID. The ECID is unique to each phone, and its signature is being checked. With this method, Apple attempts to block downgrades once newer firmware becomes available, unless you have a dump of your old firmware's unique IMG3 or signed certificate. Therefore, iBoot exploits won't be so useful for tethered jailbreaks, because such exploits will be closed in new firmwares.  Archived 2009-06-26 at the Wayback Machine.
The issue with this is that, even with the 0x24000 Segment Overflow still in bootrom, an iBoot exploit was still needed to actually flash the exploited LLB. Apple uses this ECID stuff to block downgrading iBoot to a vulnerable version. It was resolved when Geohot released his limera1n tool, which used a bootrom exploit to upload an unsigned iBoot. Still the problem remains with newer devices with the 0x24000 Segment Overflow fixed - tampering with firmware makes such jailbreak tethered unless some other exploit is used.
There are methods to help keep your downgrading ability, though.
- If it receives your ECID, saurik's servers will actively cache the necessary files. Instructions to use the servers are available.
- The SHSH associated with an ECID can be also saved by running TinyUmbrella. TinyUmbrella allows the user to restore to whatever version is associated with that SHSH file permanently. This is based on the aforementioned service Saurik provides remotely from his server(s).