I have received the "insufficient power" message on my Windows box. However, when I press the return (? right side of pearl) button when I get this message, it asks me if I want to transfer files instead of trying to charge the phone. (Don't remember the exact text of this message.) I say yes, and then I can transfer files with no problem. I'm thinking maybe it's because my computer's USB port is older. It lets me transfer files, it just won't charge the device.
Hope this helps!