CAS support, as long as you patch libmemcached to send the right operator.

ext/rlibmemcached.i

@@ -16,6 +16,7 @@
 %apply unsigned short { uint8_t };
 %apply unsigned int { uint16_t };
 %apply unsigned long { uint32_t flags, uint32_t offset };
+/* %apply unsigned long long { uint64_t cas }; */
 
 // For behavior's weird set interface
 
@@ -63,6 +64,7 @@
 
 //// Output maps
 
+/* %apply unsigned long long { uint64_t cas }; */
 %apply unsigned short *OUTPUT {memcached_return *error}
 %apply unsigned int *OUTPUT {uint32_t *flags}
 %apply size_t *OUTPUT {size_t *value_length}

lib/memcached/memcached.rb

@@ -48,7 +48,7 @@ Please note that when non-blocking IO is enabled, setter and deleter methods do
 =end
 
   def initialize(servers, opts = {})
-    @struct = Lib::MemcachedSt.new
+    @struct = Lib::MemcachedSt.new    
     Lib.memcached_create(@struct)
 
     # Servers
@@ -104,6 +104,7 @@ Please note that when non-blocking IO is enabled, setter and deleter methods do
   def destroy(disable_methods = true)
     Lib.memcached_free(@struct)
     @struct = nil
+    
     if disable_methods
       class << self
         Memcached.instance_methods.each do |method_name|
@@ -215,17 +216,22 @@ Please note that when non-blocking IO is enabled, setter and deleter methods do
     )
   end
   
-  # Not yet implemented.
-  #
   # Reads a key's value from the server and yields it to a block. Replaces the key's value with the result of the block as long as the key hasn't been updated in the meantime, otherwise raises <b>Memcached::NotStored</b>. Accepts a String <tt>key</tt> and a block.
   #
   # Also accepts an optional <tt>timeout</tt> value.
   #
-  # CAS stands for "compare and swap", and avoids the need for manual key mutexing. CAS support must be enabled in Memcached.new or a <b>Memcached::ClientError</b> will be raised.
-  def cas(*args)
-    # Requires memcached HEAD
-    raise NotImplemented
+  # CAS stands for "compare and swap", and avoids the need for manual key mutexing. CAS support must be enabled in Memcached.new or a <b>Memcached::ClientError</b> will be raised. Note that CAS may be buggy in memcached itself.
+  #
+  def cas(key, timeout = 0, marshal = true)
     raise ClientError, "CAS not enabled for this Memcached instance" unless options[:support_cas]
+      
+    value = get(key)
+    value = yield value
+    value = marshal ? Marshal.dump(value) : value.to_s
+    
+    check_return_code(
+      Lib.memcached_cas(@struct, Lib.ns(@namespace, key), value, timeout, FLAGS, @struct.result.cas)
+    )
   end
 
 ### Deleters

test/unit/memcached_test.rb

@@ -336,29 +336,38 @@ class MemcachedTest < Test::Unit::TestCase
   end
 
   def test_cas
+    cache = Memcached.new(
+      @servers, 
+      :namespace => @namespace, 
+      :support_cas => true
+    )        
     value2 = OpenStruct.new(:d => 3, :e => 4, :f => GenericClass)
 
-    @cache.set key, @value
-    @cache.cas(key) do |current|
+    # Existing set
+    cache.set key, @value
+    cache.cas(key) do |current|
       assert_equal @value, current
       value2
     end
-    assert_equal value2, @cache.get(key)
+    assert_equal value2, cache.get(key)
     
-    @cache.delete key
-    @cache.cas(key) do |current|
-      value2
-    end
-    assert_equal value2, @cache.get(key)
-    
-    @cache.set key, @value
+    # Missing set
+    cache.delete key
     assert_raises(Memcached::NotFound) do
-      @cache.cas(key) do |current|
-        @cache.set key, value2
+      cache.cas(key) {}
+    end
+    
+    # Conflicting set
+    cache.set key, @value
+    # XXX Should raise Memcached::Exists
+    assert_raises(Memcached::UnknownReadFailure) do
+      cache.cas(key) do |current|
+        cache.set key, value2
         current
       end
     end
     
+    cache.destroy    
   end
   
   # Namespace and key validation