A solution to xScope and PS CS5
If you're using xScope and Photoshop CS5, there's a good chance you've come across this annoying disappearing behavior "bug" detailed here by the Iconfactory team.
Good fortune got me into an apps and windows configuration where I could magically get the xScope tools to work as intended : a Safari window opened with "Top Sites" selected sitting under the PS doc you want to inspect. Somehow, Top Sites bugs the Photoshop call to OSX sending the PS doc behind all apps. It can either be a new Safari window or a new tab; but it can't be a blank window, it has to display Top Sites. You're then free to grab, move and adjust rulers, loupe and frames as it should be.
It's not ideal, as you have to get your apps in the proper order with repeated alt+tabs judo moves, but at least it's a solution we can work with until this odd behavior gets fixed. Hope this helps and restores sanity in your workflow.