To start, I will say we haven't run into any issues relating to consistency under load. We do have a plan for dealing with it, using an optimistic locking mechanism (where each document has a revision number and an update requires it), but as an early project we haven't had the chance to put it up against high load for a long period of time.
Also, I'm pretty confident MongoDB is going to introduce an easier mechanism for document locking in the near future.
It's difficult to find the right back-end solution that can easily scale with a business and demand. I saw this post a month ago and think it looks pretty promising and might be something you could take advantage of here. http://news.ycombinator.com/item?id=4294719
Currently it's in alpha testing and promises multi-key ACID transactions. And also introduces a really interesting layers concept. http://www.foundationdb.com/#layers
Also, I'm pretty confident MongoDB is going to introduce an easier mechanism for document locking in the near future.