I have just updated the firmware on my S450 from Tesco:
Siemen's product page has the latest software as 02139 (11/2008). This seems to be major version 2, minor version 139.
I found ref to a previous file:
http://gigaset.siemens.com/chagall/1/chagall057_01.bin
Looks like the "1" after chagall refers to the major version, as does the "01" just before the ".bin"
So the latest version (today!) is
http://gigaset.siemens.com/chagall/2/chagall139_02.bin
I entered it as the User defined firmware file under Settings, Miscellaneous and it works.
OK, it does not have the multi-account login, but there are additional features...