Looks like no 4.0 upgrade is available for 1st gen users.
I've got a 3g so some upgrades will work. As a minimum, I'm sure the unified inbox, opening email attachments, creating folders and customizing the wallpaper will work on the 3G. After this upgrade, my 3g will still be useful for quite some time. Heck, I'm happy with the phone the way it is. This will just make it better.
On a side note, I'm sure Apple could make 4.0 work across all devices, but they want you to upgrade your hardware. They are in business to make money, after all, and need to keep their shareholders happy.
You can't change your wallpaper on the 3G, sorry.
http://www.lockergnome.com/digged/2010/04/08/iphone-3g-4-0-screenshots/