(adsbygoogle = window.adsbygoogle || []).push({}); Okay, so here it goes.

1. Plug your iDevice into your computer.
   
2. Go into iTunes, then click your device.
   
3. Right, here comes the tricky part. If you remember what firmware your iDevice came with, download the original firmware from here You must login or register to view this content..
   
4. Back into iTunes, hit shift + restore. Click your original iOS
   
5. Then, when it's finished verifying, and it's installed, DON'T hit shift and restore again.
   
6. Download the desired firmware.
   
7. Next, go back into iTunes and hit shift + check for updates. This means you can choose your desired firmware.
   
8. It will think it's updating your iDevice to the latest firmware. But it SHOULD restore it to your desired firmware if done correctly.
   
9. Enjoy your custom-installed firmware!
   

This should work on all iDevices. However, this has only been tested on:
-iPod Touch 2G Non-MC
-iPhone 3GS
-iPhone 2G
-iPod Touch 3G.

It is yet to be confirmed if it works on the iPhone 4 and iPad.

Thanks,
~Josh

This works with 2G MC Models

As I've said, I've got an iPhone 3GS, iPhone 2G, iPod Touch 2G, and iPod Touch 3G, all of them are NON-MC (They are an MB!!) and it works.