On IE, a camera can be captured only once - and therefore can only be used by one tab at a time. This is because a camera can only be used by one process at a time on Windows.
On a related note, Chrome does lock the camera as well, but it shares it within Chrome’s context. So, you cannot have chrome and Skype at the same time for instance. You would also be unable to have Chrome and Firefox using the same camera at the same time on Windows.
Our plugin for Safari on Mac also uses the same mechanism of Chrome, to share 'copies' within the same browser context. This however, cannot be done from any plugin on IE because of the threading model in IE that only Microsoft has control over.
Note: The “camera” mentioned above refers to the physical devices. If you use a virtual camera (say VCam), Windows will let you have as many process as you want using it.