yep, that's it.
Writing this connected via PAN to my N900 which does GPRS (well
in the train even this is luck).
The vmware-image with scratchbox is really cool.
Added a udev-rule which whenever a bnep[0-9]* comes up
starts an instance of my userspace-nat daemon and kills
it again when the bnep-device disappears.
I think I'm gonna stay with bluetooth connection for now.
It is simple, costs comparably little CPU-Power and when
doing a terminal session to my N900 it is much less choppy
as when I do that when Laptop and N900 see each other
via a WLAN-Access-Point at home.
And what might be interesting in addition PAN leaves
WLAN unused to e.g. connect to a public WLAN-Hotspot
and share this connection.
One drawback I see at the moment is that 2 PAN clients
connected to my N900 don't see each other at the moment
as they are living each in their private userspace network-stack.
Does anybody know if it is easily possible to get the linux-bridge
to work with stock kernel N900?
Other solution would be to have on the fly interface-adding/removal
to a single instance of the userspace-nat-daemon.
By the way I can even switch back an forth between WLAN-Hotspot
and UMTS as I'm just doing while writing this text.
Good news! And bad news.
...
5. Get the IP address (yes, you get one!)
6. Try to load a web page... see the web browser show the title of the web page at the title bar... then watch the N900 reboot... ...or watch the web page load at a snail's pace, or stall, or not load at all...
How to get IP address?
I need to implement the scheme: internet wifi NAP> N800 (Easy Debian)> usbnet> PC (Ubuntu)
It may be an option Bluetooth PAN, but there is less difference in the speed of usb.
6. PC:
root@usxakov:/etc/network# dhclient -d -s 10.0.0.1
Internet Systems Consortium DHCP Client 4.1-ESV-R4
Copyright 2004-2011 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/usb0/72:2c:ae:5a:c8:76
Sending on LPF/usb0/72:2c:ae:5a:c8:76
Sending on Socket/fallback
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 3
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 8
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 7
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 9
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 12
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 13
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 15
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 20
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 20
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 17
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 18
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 11
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 11
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 11
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 15
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 11
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 16
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 15
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 13
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 11
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 15
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 10
DHCPDISCOVER on usb0 to 10.0.0.1 port 67 interval 18
No DHCPOFFERS received.
No working leases in persistent database - sleeping.
P.S.
debian-n8x0-v3c.img.ext2 - works well, but does not support vde_pcapplug (vde 2.2.3)
debian-squeeze-img.tar.bz2 - browsers do not work
ubuntu-mer-java-img-v2.tar.bz2 - no image connected
then watch the N900 reboot... 
...or watch the web page load at a snail's pace, or stall, or not load at all...