Handing off one IP address from wireless to LAN and back

[stencil]

Running Kubuntu Natty on an older (T42) IBM Thinkpad.  Both
the wired and the wireless interfaces are set to the same
manual IP address. Most of the time the wireless mode is used
but occasionally  the cable connection is needed.  Until
recently everything went well, with the wireless (eth1)
connecting automatically at startup and providing service via
the wireless router to the Internet and to the two or three
other machines on the local LAN.  Then I changed the password
on the wireless router and things fell apart.  Now:  

  --  at bootup it is necessary to provide the WEP password
via a dialogue box labeled "Secret" that pops up as soon as
the desktop stabilizes;  and

  --  the router passes data through to and from the Internet
(DSL/PPOE)  but refuses to permit connection to the LAN until
I send <ifdown> to eth0.  

The password demand is made despite the "Connect
Automatically" box being checked in SystemSettings /
NetworkConnections.  The WEP-PSK key window is always blank
-- it's not being saved, after I fill it in each time.    
Although <ifconfig> displays data on both eth0 and eth1,
/etc/network/interfaces shows entries only for the loopback
and for eth0, the LAN interface -- nothing about the
wireless.

It appears that System Settings decided to do something
creative with the networking configurations and startup
scripts -- how can I track them down and restore them to
default?  Is there some kind of cheatsheet that tells which
files are affected by SystemSettings / NetworkConnections?
Googling leads me only to advice regarding
/etc/network/interfaces, but various edits to the "auto ethX"
and "iface ethX" stanzas have not been effective.  Sigh.
Backing up and re-installing around /home is not a good idea,
unless I can be really sure that my problem doesn't lie
inside /home/.kde or something similar.  Your thoughts,
please.

gds,  stencil