Starting from iOS 4.0, Apple completely cut off the support for aging original iPhone EDGE and iPod touch first-generation. Their decision of course was justified because of the low RAM found in these first-gen devices which were released back in 2007.
But all is not lost as the iPhone jailbreak community has now come up with a mod, which brings the complete look and feel of iOS 4.2.1 for iPhone 2G, 3G and iPod touch 1G and 2G. The mod is basically based on the iPhone OS 3.1.3, to replicate the experience of iOS 4 on the first and second generation iPhone and iPod touch.
This custom firmware, dubbed WhiteD00r, includes everything from: wallpapers, new semi transparent dock (and UI), backgrounds, all new icons from iOS 4.x, Voice Control, multitasking, video recording, and more. Full list of features can be found here.
Here is a video of it in action:
WhiteD00r custom 4.2.1 firmware for iPhone 2G, 3G, iPod touch 1G and 2G can be downloaded from here.