I've done more digging and this is what I've found so far:

• The failure point is in Mongo::Connection#receive_message_on_socket
  during the method call to TCPSocket#recv (see stack trace below)
• A failure means that the code is unresponsive in that thread. It
  does not crash Ruby and you can still access the Rack web app.
• It fails on Linux but works on Windows
• Only fails if executed in Rack. Does not fail when just run directly
  as Ruby code.
• Fails inconsistently. It usually takes a few tries.
• Only fails against a remote MongoDB server, not a local MongoDB
  server
• Fails with Rack's Mongrel adapter using the provided test code.
• Does not fail with Rack's Thin adapter using the provided test code
  but DOES fail when executing in our framework. It's hard to tell what
  in our framework could cause that difference since the framework
  obviously does a lot more than Rack.
• It feels like differences in timing or combination of events is
  causing the problem since it is never consistent and changing the
  parameters around it unpredictably causes more or less
  unresponsiveness.

Stack Trace:
============================
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:704:in `receive_message_on_socket'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:615:in `receive_header'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:608:in `receive'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:399:in `receive_message'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:397:in `synchronize'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/
connection.rb:397:in `receive_message'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/cursor.rb:
339:in `send_initial_query'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/cursor.rb:
316:in `refill_via_get_more'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/cursor.rb:
304:in `num_remaining'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/cursor.rb:
61:in `next_document'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/db.rb:
442:in `command'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/gridfs/
grid_io.rb:319:in `get_md5'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/gridfs/
grid_io.rb:310:in `to_mongo_object'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/gridfs/
grid_io.rb:169:in `close'",
"/usr/lib/ruby/gems/1.8/gems/mongo-0.19.1/lib/../lib/mongo/gridfs/
grid.rb:60:in `put'"

Rack Script (same as previous post):
=============================
require 'rubygems'
require 'rack'
require 'mongo'
require 'logger'
proc = Proc.new do |env|
request = Rack::Request.new( env )
response = Rack::Response.new
connection = Mongo::Connection.new( "10.1.9.1", 27017, :pool_size =>
10, :timeout => 2, :logger => Logger.new( "mongo_rack.log" ) )
db = connection.db( "Production" )
grid = Mongo::Grid.new( db )
id = grid.put( 'a' * 1000000, "file.txt" )
response.write( "This is the file id: #

" )
response.finish
end
app = ( Rack::Builder.new do
use Rack::Lint
use Rack::ShowExceptions
run proc
end )
Rack::Handler::Mongrel.run( app, :Port => 9000 )