I had this problem and found a solution:-
I use two monitors, and every time I opened IE8 Dev tools, the dev tool window would either be maximised to full screen or minimised to the task bar (irritating to say the least). The other problem was that when maximised, the window was locked into the full screen mode and could not be dragged to a more convenienet size & position. so it was all or nothing !
To get round this I "stumbled upon" a fix that now allows me to open the Dev Tools window as it should be (a seperate window that can still be positioned & sized how I want it)....
Press ctrl+p (PIN) to attach the dev Tools window to the bottom of the actual browser window. At this stage move your browser window into your main monitor if its not already there. Then un-pin it. For me the Dev tools window then started to behave properly and consistently.
Why the hell MS hasnt released an update for this yet is beyond me. Any way hope this helps any of you.