My Solution:
Although not optimal, casting the input as the correct type seems to give the runtime binder enough of a tip that it can resolve the correct method signature to use.
For example, here is some "failing" code that threw the above exception (from an MVC4 partial - code is from a contractor and I'm not proposing this is the best way to create an anchor tag - yet it does illustrate the specific issue nicely):
<a rel="nofollow"
href="https://twitter.com/share?text=Check%20this%20out!%20@(ViewBag.Title)%20
@(HttpUtility.UrlEncode(Request.Url.AbsoluteUri))%20%40codinghorror"
target="_blank">
twitter
</a>
And here it is, rewritten slightly with casting:
<a rel="nofollow"
href="https://twitter.com/share?text=Check%20this%20out!%20@(ViewBag.Title)%20
@(HttpUtility.UrlEncode((string)Request.Url.AbsoluteUri))%20%40codinghorror"
target="_blank">
twitter
</a>
After this fix, I reloaded the offending page and sure enough, POOF!, it worked.
Happy Trails!