Same thing has happened to me. I purchased and installed Mtn Lion (on this MacBook Pro). I even installed it on my home Mac mini. However, when I went to the App Store, it was not listed it in my Purchases list, and it has a purchase price button on the Mtn Lion app page. I restarted the computer(s), signed out & back in w/ correct Apple ID, trashed preferences, asked it to check for unfinished downloads, tried on other computers (one w/ Mtn Lion, one w/ Lion installed)...nothing worked so far.
Then I stumbled on the answer...
Go to the top Menu bar > Store and click on View My Account (accountname), then enter you password. In the following dialog window, you should see listed Hidden Purchases under the iTunes in the Cloud section. Click on the View Hidden Purchases and the next screen should show Mountain Lion as hidden...simple click Unhide. It should now show in your Purchases list as well as offer Download button on Mtn Lion info page.