I like the idea in general but I can see myself typing "@foo" sometimes without intending to be referring to a person since I blog about programming languages and adjacent topics sometimes. As suggested, this would make the markup nondeterministic depending on whether foo happened to be the name of a user at that particular time, with unpredictable results.

I think it's fine in markdown format but have the above concern about it appearing in other formats. Perhaps if it was never expanded if inside other tags like <tt>@foo</tt> or <pre>@foot</pre>, which would at least protect the use case I have in mind?