Everyone is correct in the use of @Html.Raw() but I want to point out to be careful with this, as it can make your site susceptible to XSS vulnerabilities.
I would combine the @Html.Raw(ViewBag.SomeMessage) with Microsoft’s Anti-XSS Library to make sure you do not introduce any vulnerabilities from this.
Edit:
The advantage of the Anti-XSS library (if you haven’t looked at it) is it has a whitelist of approved markups (such as <b>, <h3>, etc..) so that only approved markups will be un-encoded.
Edit2:
Here’s an example of how this is done.