While viewing my purchased apps in the Mac application of 'App Store' I clicked the (X) next to the iPhoto Installed button. iPhoto immediately disappeared from the list of purchased apps.
There is no Undo for this nor any other obvious or searched solution. Here is how to restore the listing of purchased apps. It is not obvious.
Delete an app if you haven't. Under the App Store menu item Store drag down to View My Account.
Enter your password and a new option is available in the account window listing Hidden Purchases with an option to unhide them.
You do not see this option unless you have deleted at least one list item. This provides a way of filtering out rejected apps you have purchased.
[crarko adds: I tested this (in 10.6.8), and it works as described. At least with the iTunes App Store, there is a warning dialog and some instruction when clicking on the X to hide an app as mentioned in this hint.]
Mac OS X Hints