I've compared two Openlayers samples:
First one zooms in smoothly, second one just flashes and shows tiles from next zoom level.
The only difference between those two versions is the library - release dev example (second) uses default multi-file version, while the first one - uses some compiled version (not sure from which tag).
Question is: how to enable smooth zoom in Openlayers 2.12? Or how to enable smooth zooming in any Openlayers version ;)
The fluent zoom feature is included by default in 2.13 library: