-snip- although I can't say for sure in which version the encryption feature appeared in the Bitcoin client.

I did some digging to answer that:
It's implemented in version 0.4.0, check the date here: https://github.com/bitcoin/bitcoin/tags?after=v0.5.0rc1
Here's the "readme" (release notes) at that time: https://github.com/bitcoin/bitcoin/blob/v0.4.0/doc/README#L28
The wallet encryption part isn't present in the previous release version 0.3.24.