Hi Simon,
The CWIC browser plugin includes a "system lock" to prevent concurrent phone registrations from the same machine: after registering from one browser (ex: Firefox), attempting to register from a second browser (ex: Chrome) leads to the "already logged in" error. That said, browsers have different process models, which can cause different behavior as you described.
Firefox, Chrome and Safari will load one single instance of the plugin: after registering from one tab, other tabs will see the CWIC phone regsitered as they access the same instance.
Internet Explorer implements a different model, it is not guaranteed tabs will share the same CWIC plugin instance. In your case, the 2 IE9 tabs loaded 2 distinct plugin instances. First tab registered ok, but second instance could not acquire the system lock and raised the "already logged in" error. Same as if one Firefox tab is registered and a Chrome tab attempts to register.
Hope this makes sense, Thomas

Hi Simon, I think IE uses the "TabProcGrowth" registry key to configure how IE launches new processes when opening new tabs. http://blogs.msdn.com/b/askie/archive/2009/03/09/opening-a-new-tab-may-launch-a-new-process-with-internet-explorer-8-0.aspx We never tested a customized TabProcGrowth with the CWIC plugin, you might want to give it a try. Also not sure if it can impact other web sites/applications. More information about LCIE (Loosely Coupled IE) can be found here http://blogs.msdn.com/b/ie/archive/2008/03/11/ie8-and-loosely-coupled-ie-lcie.aspx Thomas