Ironically the GUI of guiminer.exe (2011-07-01) disappeared from my Windows 7 x64 machine.
After investigating, I found out that poclbm.ini's settings for "window_position" were changed to -32000 for both horizontal and vertical positions. I reset them to 0 and I could see the GUI again when launching guiminer.exe. Hope this helps other people. You may want to check why it resets to -32000 instead of 0. It happened a second time to me just now, while I was opening and closing guiminer.exe to test it. Here's the ini content:

    "window_position": [
         -32000,
        -32000,
        160,
        27
    ]