I heard you like title attributes, so I put more code in your code.

app.rb

@@ -7,7 +7,26 @@
 require 'sinatra/reloader' if development?
 
 get '/' do
-  @attendees = CSV.read("rcx.csv")[1..-2].map{|a| a[2][1..-1] if a[2] }
+  @attendees = AttendeeList.new("rcx.csv").with_twitter
 
   haml :index
-end
+end
+
+# Sinatra::Reloader can't reload Struct subclasses :(
+if defined?(AttendeeList)
+  [ :AttendeeList, :Attendee ].each { |c| Object.send(:remove_const, c) }
+end
+
+class AttendeeList < Struct.new(:path)
+  def all; CSV.read(path)[1..-1].map { |row| Attendee.new(*row) } end
+  def with_twitter; all.select(&:twitter?) end
+end
+
+class Attendee < Struct.new(:first_name, :last_name, :twitter)
+  def full_name;   [ first_name, last_name ].compact.join(" ") end
+  def twitter?;    !twitter.nil? && twitter.length > 0 end
+  def username;    twitter[1..-1] end
+  def url;         "https://twitter.com/#{username}" end
+  def image_url;   "http://img.tweetimag.es/i/#{username}_b" end
+  def description; "#{full_name} (#{twitter})" end
+end

views/index.haml

@@ -8,7 +8,5 @@
   %body
     - for attendee in @attendees
       %article
-        - if attendee
-          %a{href: "http://twitter.com/#{attendee}", rel: 'external'}
-            %img{alt: attendee, src: "http://img.tweetimag.es/i/#{attendee}_b"}
-
+        %a{href: attendee.url, title: attendee.description, rel: 'external'}
+          %img{alt: attendee.description, src: attendee.image_url, height: 73, width: 73}