ah, thanks, that helped already a few steps further.

connection appears as WLAN infrastructure -- has to be set to Ad-Hoc to appear in networks list.
yet, when connecting to that network, routing via my computer (the one the n900 is connected to via usb) doesn't work anymore.

values set here:
address 192.168.2.15
netmask 255.255.255.0
gateway 192.168.2.14

to be clear: routing, dns and so on work when connecting the old way (but only in a terminal).
the moment i connect to the new "USB network", it stops.
i run a ping to an external ip address, that works -- then i hit "connect" and after a moment (when the connection icon stops blinking), the ping stops, too.

apparently i still miss something here.

EDIT:
and here's another one -- when i remove the stanza from /etc/network/interfaces, usb0 _never_ comes up! the configured "USB Network" brings instead the wlan device up -- which doesn't help ...
why is thinks icd2 (or whatever manages those connections), it si a WLAN connection?
the gconf command clearly states "USB":

gconftool-2 -s -t string /system/osso/connectivity/IAP/USB/type USB