Release all the code to the community.
OpenERP uses own library for web client and some other open source libraries. They share full code with community (As similar as wordpress and drupal).
Include a link in our application to Openerp.com
If you use existing web client of OpenERP for online SAAS keep powered by OpenERP in screen anywhere, you might customize Login screen or web client,
then just keep powered by openerp on screen. rest of all header/footer/head title any things you can change, no one can make argument.
Display the name or logo of Openerp.
Do not hesitate to change logo, change logo anytime it based on your own business how to want to manage it.
Just keep powered by OpenERP as it is for SAAS, Ya even that can remove if end client wants to remove in their separate solution, its upon them.
Just keep "Powered by OpenERP" on anywhere on screen before login and after login. Else do any things changes as your requires.
Also If you create separate customization code and separate functionality for login screen, then no such "Powered By OpenERP" require to keep on that screen,
because you become owner of that screen and code customization.
Finally if separate web client is designed buy you or your company for android or web or iphone no bit "Power by OpenERP" is require any where, even in yours code.