Michael Grothaus of TUAW on an enhancement in OS 10.7.2:

A final tweak I’ve noticed is that now you can remove purchases from your Purchased history in the Mac App Store. Click the Purchased button at the top of the store then move your cursor over an app from your purchase list. You’ll see a small X appear at the far right. Click it to delete the app from your purchase history. Deleting a currently-installed app from your purchase history won’t delete it from your computer.

It’s about freaking time.