Unfortunately no real life implementations for me as yet. I guess because it is fairly "new" vendors are still putting it through it's paces and testing. One example would be that some trading applications use FEEDS of data. I know of one client that is hosting the feeds in a conventional database. Since this data changes fairly often and redundancy is not critical, an in memory db will be a good solutions. They will still be able to do what ever they are doing in the database but with much better performance.
I am sure there will be a few more examples of an I.M.D.B, just can't think of them.