I have now similar problem with connection to 3Com OfficeConnect Wireless 11g router. The connection was fine before but in the last two weeks I have always got an error message "Unable to retrieve IP address" whenever I have tried to connect to it. However, I can connect to this router with my laptop. The router do not have enabled any security method.
I do not have any troubles with connection (N900) to other routers secured with WPA or WPA2.