Can I cancel a download of a Mac Store app?
Specifically in Lion, here's how to do it:


Go to Launchpad, click and hold on the downloading item
All icons will start wiggling
Click the (X) on the download


or 


Hold down the Option key from the “Purchases” menu in the Mac App Store, this causes the “Pause” button to turn into “Cancel”. See this article for detail.


It will ask for a confirmation. I confirmed delete, and it was still there    - a reboot made it disappear, but I suppose you could restart just Launchpad as well.