Você só precisa armazenar uma instância de datetime.datetime.
Inserindo a partir do shell python:
>>> c.test.test.insert({'date': datetime.datetime.utcnow()})
ObjectId('4e8b388367d5bd2de0000000')
>>> c.test.test.find_one()
{u'date': datetime.datetime(2011, 10, 4, 16, 46, 59, 786000), u'_id': ObjectId('4e8b388367d5bd2de0000000')}
Consultando no shell do mongo:
> db.test.findOne()
{
"_id" : ObjectId("4e8b388367d5bd2de0000000"),
"date" : ISODate("2011-10-04T16:46:59.786Z")
}