In which version of Android was encryption of /data made available? These docs suggest that it has been available (only) since Android 3.0 Honeycomb. By contrast, conversation on discussion boards suggests it was available in 2.3.4 Gingerbread.

So, what is the answer to the titular question?

Bonus points if your answer clarifies the following. Are the official docs misleading? Or were those Gingerbread devices instead released after Honeycomb, and loaded with some kind of backported encryption feature? Or were those Gingerbread devices instead loaded with some kind of encryption feature added by the device manufacturers, that wasn't part of Android? In short, what gives?
Gingerbread 2.3.4 ROM added support for device encryption, but the feature wasn't accessible from settings until Honeycomb (tablets) and ICS (smartphones).

To enable device encryption in Gingerbread device, you need to add an Exchange account. Not sure, but it should be accessible through API too.

There are also some hacks to enable device encryption directly in Gingerbread: http://forum.xda-developers.com/showthread.php?t=1405165