The AppStore (I'm using Snow Leopard 10.6.8) now has the ability to hide purchases: just go to the PURCHASES tab and click the little X that appears when you hover over the right-hand side near INSTALLED.
You can make the app reappear in your PURCHASES tab by selecting VIEW MY ACCOUNT in the STORE menu item, then select VIEW HIDDEN PURCHASES.
I'm sure other folks' purchase lists were getting as gnarly as mine as I've tried a few titles then uninstalled them.