Pre-order bonuses aren't going anywhere, it seems. And if that truly is the case, the least a developer can do is make retailer-exclusive rewards available after the fact. I'll never buy this stuff, but someone else presumably always will as evidenced by the practice. Bethesda plans to bundle four Dishonored packs on May 14 (and May 15 for European PS3) at a price of $3.99.
In general, we're looking at in-game bonuses for various abilities plus other trinkets -- so nothing that can't be skipped -- but there's a breakdown of the specific content over on the Bethesda Blog should you be curious. From the sound of it, though, The Knife of Dunwall should have your attention, not this. Go play that instead!