The good news today is that promo codes for iOS apps are no longer limited to the U.S. App Store, but it looks like the upcoming Mac App Store (set to launch in January 2011, even though someone says it may open as early as next week) will be based on a different system. In fact, it appears that developers won't have the possibility to generate promo codes for Mac apps sold through the Mac App Store.
In the iTunes Connect Developer Guide (version 6.2, last updated on November 1, 2010 - PDF) Apple mentions that promo codes are exclusive to iOS and won't be available for Mac apps:
Promo Codes button (for iOS apps only). See the Requesting Promo Codes section to learn about promotional codes)
Promotional codes are not available for Mac OS X apps.
We have also looked in the Mac Development forums (Mac App Store section), Review Guidelines and Mac App Store Submissions FAQ -- there's no mention of promo codes for Mac apps. The iTunes Connect Developer Guide was last updated on November 1st, a few days after the Mac App Store announcement at the Back to the Mac event, so it's possible that Apple will revise its position by the new Store opening. We find quite strange, though, that several submission details and tips have been provided, but promotional codes for Mac apps didn't make the list.
The lack of promo codes for Mac App Store apps would make sense considering that Apple won't accept trials, demos and betas either. If true, this would turn the Mac App Store into a rather simple buy -> download purchase system, although it's obvious that users will still have the possibility to redeem iTunes Gift Cards. The big deal is for developers, who won't be able to gift apps sold in the Mac App Store to their users / press.
We hope Apple will change this by the official opening of the Mac App Store. We're also hoping Apple will introduce promo codes for in-app purchases someday.