You can have up to 9 home screens.
1- Download and install modified hildon desktop from here.
It may take several minutes to download and install packages (5min or so...)
2- Reboot your N900
3- Open a terminal and type the following commands :
Sudo gainroot (then, Enter key)
gconftool -s /apps/osso/hildon-desktop/views/5/bg-image -t string "/path/to/background.jpg"
gconftool -s /apps/osso/hildon-desktop/views/active -t list --list-type int [1,2,3,4,5] (then, Enter key)
With this example you have 5 home screens! Works perfectly. Have fun with up to 9 Home Screens!