I'm currently working on an applet, and testing on OS X. I set up a mouse look system using the mouse grabbing system and Mouse.getDX()/Mouse.getDY() and it worked fine in eclipse.
When I uploaded it to the server and tried to test it in a webpage Mouse.getDX()/Mouse.getDY() simply return 0 most of the time. The only way it responds is if you click (triggering mouse.grabbed(true)) while moving the mouse. It seems to actually get the very first sample correctly right as you click but after that it doesn't respond.
I tried the applet in Safari and Firefox and it doesn't respond. Annoyingly, in Safari it also doesn't get the mouse button release if the cursor exits the window as well.