A further update on this issue.
I am consistently experiencing this issue on 1.8RC5 (but as previously noted, I have always had the same problem going back to 1.7, and the 1.8 version is a separate installation under a different directory).
I now noticed that I have the same problem with other saved processes that have a dynamic interface; e.g. If I save a dynamic crop as an icon. save everything as a project, quit PI, restart (e.g. next day), double-click the process icon and I will get the crop box displayed on the current image but the actual dynamic crop interface is not displayed.
I have looked in all the obvious places (e.g. other workspaces, other monitor, on the floor under the desk, scrolled further down in the window, etc.) The only way to exit dynamic mode is to click the 'close' checkbox to quit PI. It then asks if you want to quit the dynamic process (yes), and then warns if you have unsaved changes (clicking no at this point means you can stay in PI and regain control of the interface).
Everything works fine if you open a new instance of the relevant process from the menu, select your target image and then drag and drop your saved process icon to the control bar.
The machine does not change resolutions as it is a desktop setup with two monitors in extended desktop mode. Running on Windows 7 Home Premium 64 bit.
It seems likely to me that the interface dialog is being displayed somewhere that is not visible, but can't figure out where or why.