This might be a little verbose but I had to spend some time with the Rails source code to learn how the caching internals work. Writing things down aids my understanding and I figure that sharing some notes on how things work can't hurt. Skip to the end if you're in a hurry.

Why It Happens

This is the offending method inside ActiveSupport:

def should_compress?(value, options)
  if options[:compress] && value
    unless value.is_a?(Numeric)
      compress_threshold = options[:compress_threshold] || DEFAULT_COMPRESS_LIMIT
      serialized_value = value.is_a?(String) ? value : Marshal.dump(value)
      return true if serialized_value.size >= compress_threshold   
    end
  end
  false  
end

Note the assignment to serialized_value. If you poke around inside cache.rb, you'll see that it uses Marshal to serialize objects to byte strings before they go into the cache and then Marshal again to deserialize objects. The compression issue isn't important here, the important thing is the use of Marshal.

The problem is that:

Some objects cannot be dumped: if the objects to be dumped include bindings, procedure or method objects, instances of class IO, or singleton objects, a TypeError will be raised.

Some things have state (such as OS file descriptors or blocks) that can't be serialized by Marshal. The error you're noting is this:

can't dump hash with default proc

So someone in your model has an instance variable that is a Hash and that Hash uses a block to supply default values. The column_methods_hash method uses such a Hash and even caches the Hash inside @dynamic_methods_hash; column_methods_hash will be called (indirectly) by public methods such as respond_to? and method_missing.

One of respond_to? or method_missing will probably get called on every AR model instance sooner or later and calling either method makes your object unserializable. So, AR model instances are essentially unserializable in Rails 3.

Interestingly enough, the respond_to? and method_missing implementations in 2.3.8 are also backed by a Hash that uses a block for default values. The 2.3.8 cache is "[...]is meant for caching strings." so you were getting lucky with a backend that could handle whole objects or it used Marshal before your objects had hash-with-procs in them; or perhaps you were using the MemoryStore cache backend and that's little more than a big Hash.

Using multiple scope-with-lambdas might end up storing Procs in your AR objects; I'd expect the lambdas to be stored with the class (or singleton class) rather than the objects but I didn't bother with an analysis as the problem with respond_to? and method_missing makes the scope issue irrelevant.

What You Can Do About It

I think you've been storing the wrong things in your cache and getting lucky. You can either start using the Rails cache properly (i.e. store simple generated data rather than whole models) or you can implement the marshal_dump/marshal_load or _dump/_load methods as outlined in Marshal. Alternatively, you can use one of the MemoryStore backends and limit yourself to one distinct cache per server process.

Executive Summary

You can't depend on storing ActiveRecord model objects in the Rails cache unless you're prepared to handle the marshalling yourself or you want to limit yourself to the MemoryStore cache backends.

The exact source of the problem has changed in more recent versions of Rails but there are still many instances of default_procs associated with Hashes.