Given /^I am browsing the site using Safari$/ do
  header "User-Agent" , "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_6; en-us)"
end

Like me, you may have found out the hard way that this doesn’t work. Webrat does not automagically apply these new HTTP headers to your request – they certainly don’t make it to my controller. What worked for me:

Given /^I am browsing the site using Safari$/ do
  headers["User-Agent"] = "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_6; en-us)"
end
 
When /^I visit my precious site$/ do
  get '/my/precious/path', my_query_string, headers
end

In the code above, headers is a method call that returns all the HTTP headers for your request. Just tack headers on as the third argument of your request, and you’re good to go.