Can an app that was removed from the Mac App Store be re-downloaded? I do a clean install whenever I upgrade my OS to a new major release.

I know Coda 1 is no longer in the App Store and isn't supported by Panic anymore but I'd like to keep using it because it works for my needs. I am curious whether Coda 1 can be re-downloaded on the Mac App Store under "Purchases" after I install a clean OS (particuarly Mountain Lion later this month)?

If not, can I copy the App to a USB drive and put it manually in to a cleanly installed OS and will it work?
Yes. Most cases the developer will leave the app on the store, but just remove it from being sellable to new owners.

That allows them to submit bug fixes for existing customers, not force them to upgrade, allow them to re-download things - but not have new buyers buying the wrong version (not the latest).

Of course, the developer or Apple can also remove the app entirely - but this is far less common. What is common is when the developer keeps the same name as far as the store is concerned. In this case, you'll want to have a backup that you control to re-install no matter what Apple or the developer chooses.