Glad to hear that switching to Mongo worked out for you. I remember talking to you a while ago when most stuff was running on SqlServer.

For our project we went the Windows Azure route and store our 'events' in their NoSql Table Storage. I must admit I have been tempted to switch to MongoHQ due to the lack of query support from Table Storage. Funny enough, we're also working on a custom leaderboards feature for our customers and the lack of secondary indexes really makes it difficult.