Newer devices like the i Pad 2, i Pad 3, i Phone 4S, i Phone 5 and i Pod Touch 5G has different restrictions however, because they are not vulnerable to the limera1n exploit and are governed more-so by APTickets.
For the most part, older i OS devices with an A4 chip can be downgraded to any firmware version as long as you have saved SHSH blobs and APTickets.
This includes the i Phone 2G, i Phone 3G, i Phone 3GS, i Phone 4, i Pad 1G, i Pod Touch 1G, i Pod Touch 2G, i Pod Touch 3G and the i Pod Touch 4G.
Once you have your SHSH blobs signed, as stated previously, if you upgrade to a new firmware version and decide you don’t like it you can downgrade back to the firmware version which you have saved SHSH blobs for.
There is unfortunately some restrictions that need to be kept in mind though.
APTickets are much more secure however, because when you try to restore or upgrade your i Phone, i Pod Touch or i Pad a request is sent to Apple’s servers to confirm the APTicket.