I\'ve found this question answered for C# and Perl, but not in the native interface. I thought this would work:
db.theColl.find( { _id: ObjectId(\"4ecbe7f9e8c
Even easier, especially with tab completion:
db.test.find(ObjectId('4ecc05e55dd98a436ddcc47c'))
Edit: also works with the findOne command for prettier output.
findOne