An ongoing issue with Adobe Photoshop CS5 that a few people are experiencing is the menu items will not work and instead will appear greyed out after a while of using the program. In some instances only a subset of menu options will be inactive, but in other instances the full menu set will not work.
The problem seems to be intermittent for most people, and happens randomly after using the program for a specific amount of time. Some have reported that it seems to happen after performing specific functions like saving, but for others the problem so far has not been associated with any specific cue.
Unfortunately there are no lasting fixes for this problem, and currently people have only had success by saving their work and relaunching the program or the whole CS5 suite if other CS applicaitons are open. According to Adobe employee "Chris Cox" in a recent forum posting, the problem seems to be with how Photoshop handles system events in Apple's Cocoa API, which essentially means the problem is with how Photoshop is handling input from mice, keyboards, tablets, and other devices.
While it is great Adobe is aware of and attempting to tackle this issue, for now the problem is intermittent enough that even Adobe initially found it difficult to debug; however, according to Jeff Tranberry from the Photoshop development team apparently they have pinpointed the problem and are working on a fix. An update or workaround to address this issue may arrive soon enough, but for now if you are experiencing this problem you should be able to resort to quitting and relaunching your CS5 applications.
Beyond quitting, you can try tackling the OS's input features, which may include trying different USB ports for your keyboard and mouse, updating or uninstalling third-party driver software (especially for tablets and other specialized input devices), locating and removing any input managers you may have installed, and unplugging multiple input devices if connected.
In addition to testing your system's input setup, you can try more global approaches like reapplying the latest system combo update, uninstalling and reinstalling Adobe CS5, and trying CS5 on a fresh installation of OS X and migrating to that if everything works properly. If you have an external hard drive you can install OS X to that and then just install CS5 on that drive to test the installation out.