I'm working on a single page app that's about 30% Rails and 70% Coffeescript/Backbone.js. Test::Unit is practically useless for us since users never hit any plain HTML pages besides the login page.
Imagine the current HTML5 Pandora having bugs with one particular song in Chrome only. How do you test for that using Test::Unit?
there are a bajillion libraries out there for unit testing javascript (something test::unit-ish would be qunit). rails + backbone + JST style templating actually makes TDDing your js relatively painless
We're looking into integration-level testing for our Javascript since it'd give us full coverage of the bugs we see.
I forgot to mention we also use Socket.io for real time push updates to user Backbone models. A good number of bugs don't crop up until another user modifies data and those changes get pushed out to another person.
Testing with front end javascript is more difficult due to different interpreters, much more variation in environment, etc. That doesn't mean you can't have tests for your core business logic at least in something like Vows or Mocha.
Just use Test::Unit and move on with your life. Write some tests. That's what counts.