How do I find out the status of a large download from the Mac App Store?
Aha... found it. In the App Store application menu, you can choose Store > Check for Unfinished Downloads which pops up a window like this:
You can also just check the Purchased tab.
It's a bit of a hack, but you can check the size of ~/Library/Application Support/AppStore with Get Info in Finder -- assuming it's Lion that you're downloading, it should be about 3.5GB in total (I'm currently on 444MB, so it will take me a while).