Found my own answer...
I've revised my Rails and Bootstrap tutorial and updated the rails-bootstrap example app accordingly.
The Rails flash message hash contains a key (a "name") and a value (the "message").
Under Rails 4.0, the key is a Symbol.
Under Rails 4.1, the key is a String.
Styling flash messages for display with Bootstrap or Foundation requires parsing the key to determine if it is an alert or a notice.
Under Rails 4.1, names are Strings and are not matched by the code above, instead getting styled in red with the alert-danger class.
To fix it, the code for displaying flash messages with Bootstrap should change to accommodate both Rails 4.0 and Rails 4.1:
<div class="alert alert-<%= name.to_s == 'notice' ? 'success' : 'danger' %>">
For Foundation, the code should change to this:
<div data-alert class="alert-box round <%= name.to_s == 'notice' ? 'success' : 'alert' %>">