There's a way to do it, but it doesn't exactly work "on the fly." You have to do some modifications to get it to work right, so changing the background on the fly like on the flip background for all pages, doesn't work as of yet. (to my knowledge)
Here's the original post at XDA:
http://forum.xda-developers.com/show...2&postcount=50
I'm thinking I might start a thread and start helping people with these, as it's quite... complicated to do... but it only works with 512x512 pictures. But that's better than nothing!
Basically, there's a background for all tabs manila file, by default it's a very small picture and it doesn't contain anything, but it can be modified to contain a picture and increased in size, which is what the process above does. I made mine into a cab file so I don't have to use Total commander to copy it in manually.
I'm thinking I might just start a thread and start making cabs for people to simply download and install as they choose.
If someone knows of a way to get it to work by simply changing the wallpaper in the setting screen, let us all know...