When running a mapreduce that looks like this:
coll.mapreduce(map('uuid'), reduce, {:out => { :replace => scratch, 'db' => 'dp'},
:query => {:uuid => r}.merge(ts)})

I get this:
/Users/nacuff/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.5.1/lib/mongo/collection.rb:64:in `initialize': new_name must be a string or symbol (TypeError)
from /Users/nacuff/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.5.1/lib/mongo/db.rb:314:in `new'
from /Users/nacuff/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.5.1/lib/mongo/db.rb:314:in `collection'
from /Users/nacuff/.rvm/gems/ruby-1.9.2-p180/gems/mongo-1.5.1/lib/mongo/collection.rb:627:in `map_reduce'

removing the 'db' (or :db) key makes it work just fine.