Greek characters not preserved correctly

When editing a page, I wanted to enter a greek character (Sigma – Σ). I did this by entering the character using my UTF-8 enabled keyboard. This actually worked - I saw the Sigma character in the rendered wiki page. However, during a subsequent edit on a different part of the page, the character seems to have been (attempted to be) converted to an HTML entity:

Σ

Unfortunately, the HTML entity is not displayed correctly – it simply appears as the entity text itself, not as the intended character.

Repro:

1. Using source view, add this to a wiki page:
UTF-8: Σ æ ø
2. Save and observe that the characters render correctly.
3. Open the page for editing and CKEditor, and save without changing anything.
4. Observe that the sigma character has been rendered as the text:

Σ