I don't see a lot of copies of iWork, iLife, Apertuer, etc, being pirated now. DVD's are much easier to make dmg's of and pirate. Having purchased software via the App Store, and experimenting out of curiosity, It's not so easy. There's code buiried that ties the install into your App Store/iTunes account. I can't find it. But, I'm sure eventually someone will. All those bootleg copies of App Store software and iTunes movies all over the place already and all that... not.
The app store is not a simple download of an app, it actually is compiling files during download and embedding your Apple ID into the software, matching it to your registered machine ID's. Very difficult to defeat. Much more than DVDs. So unless you purposfully give someone your Apple ID for piracy purposes, well...