Hi!
Maybe you wanna try the sane and xsane packages I compiled for Maemo.
Only the net backend is available. Others don't make a sense.
However the xsane window does not fit the screen.
Feedback is welcome.

I had to install it with apt-get install xsane since the app wasn't shown in my app.manager.

But how can I manually enter a scanner's IP (it is a HP Officejet already working with Ubuntu's Sane). When I try to read the instruction in the app's help section unfortunately the line containing the command isn't displayed completely.
I only can read a man sane-"backendr... rest unknown.

You only have to add the servername or IP to /etc/sane:d/net.conf.
Also make sure, that net is activated in dll.conf.
xsane and sane is only in the extras-devel repository.

My Ubuntu laptop connected to the WiFi shows the scanner in the terminal:

kyros@ky_user-laptop:~$ scanimage -L
device `hpaio:/net/Officejet_Pro_L7700?ip=192.168.1.108' is a Hewlett-Packard Officejet_Pro_L7700 all-in-one

But I I couldn't get it recognized by the N900 which is connected to the same WiFi network. I tired to follow the lines according to http://www.mail-archive.com/hplip-he.../msg03409.html in section SANE CONFIGURATION but adding hpaio to dll.conf and hpaio:/net/Officejet_Pro_L7700?ip=192.168.1.108 or simply 192.168.1.108 to net.conf didn't help...

What shall I insert into the dll.conf and net.conf?

On the phone only "net" works. To net.conf on the phone you only have
to add the servername or the IP.
You have to add the hp stuff (or other backends) to the server.
Does your scanner work on other clients in the network?
On the server I have to run same as root to work at all.

Hmmm... there is no such thing like a server which runs the scanner in my network. The HP Officejet Pro L7780 is connectected via it's WiFi interface to my home WiFi network (IP 192.168.1.108) which is broadcasted by the router (IP 192.168.1.1). Usually I install the hplip client application on the linux clients including printer driver, sane scanner app, fax and so on. So I thought that xsane on the N900 could find the device's scanner when connected to the WiFi. Unfortunately it seems that it's not possible without hplip app.

That would be the net-backend I think.