Yes, I have come across the same issue and it happens to all special HTML characters. I logged this issue with EPiServer and they have identified it as an issue and assigned it issue number 45513. Their response below: "Thank you for the report. We agree that this is an unwanted behaviour and I have added the following issue for this: #45513: The dhtml editor does not html encode all characters. However, you should be aware that this could be behaviour in the edit functionallity in IE that the editor builds on. If this is the case, it is not sure that we are able to change this behaviour but we will at least investigate this."

Have you ever seen it where instead of the symbol (such as the trademark symbol) when viewing the page a question mark symbol is displayed instead. I've checked the HTML source of the page when viewing it and there is a question mark in the HTML source but when looking at the HTML source in edit mode, the trademark symbol is in the code. I'm baffled by this one as I can paste the symbol in to the sample site and it appears ok. In both places we're using the XHTML string property.

No, I have not seen this before. Have you tried viewing the content in different browsers? It could be a doctype/encoding issue that the browser can't understand, but I can't really be sure.