Whilst I'm 100% sure Apple would love for you to purchase the new phone, I'm pretty sure they would have included custom wallpapers on the 3G if it was feasible. If they just wanted you to upgrade, they wouldn't give you the update at all.
Remember, custom backgrounds effectively means loading a 1mb image constantly. For a 128mb device, that's quite a lot. I think Apple put the smoothness of the overall experience above a custom wallpaper.
And no, I'm not being a fanboy. Go and jailbreak them if you want it. Yes it works, but it's just reducing the amount of RAM available to apps on a very RAM-limited system. Go on jailbreakers - see how much free RAM you have with macrumors.com loaded in Safari. Then you see how valuable RAM is on the 3G.