T4MVC generates code to invoke the wrong method overload for the ActionLink extensions that take an htmlAttributes parameter.
Although the links are correct, attributes with underscores are not correctly transformed to use dashes before being rendered (e.g. data_id does not become data-id).
I've modified my local .tt file with a fix. You can see the code changes made here: http://stackoverflow.com/questions/9593631/is-usage-of-html-5-data-attributes-broken-in-asp-net-mvc-4-beta