With Snow Leopard and Lion, you could hover your mouse over the ‘Install’ or ‘Download’ button in the App Store Purchases list, then an ‘x’ would appear allowing you to ‘HIDE’ this purchase from the list. This was useful if another user on your computer opened up the App Store and looked at what apps were purchased on the computer.
Mountain Lion now has a slight tweak to this. You now need to Control-click or right-click the App you wish to hide and choose ‘Hide Purchase…’ then confirm by selecting ‘Hide Purchase’ in the popup window that appears :
To unhide an item, click on the Account link on the main page of the App Store, (you may need to re-enter your AppleID password), then go to iTunes in the Cloud > View Hidden Purchases :
You can then select ‘UNHIDE’ for the purchase you wish to unhide :
This feature has been tested using OS X v10.8.2 which was the latest Mac OS release at the time of writing and also using the latest Internal and external Apple keyboards.