We have been running a mongod process since March 28. I've noticed entries like the following since April 8. The log has been rotated, but it's possible the warnings started appearing on April 7.

Thu Apr 12 15:01:38 [conn89420] warning: virtual size (46611MB) - mapped size (41518MB) is large (5036MB). could indicate a memory leak

We have Mongo monitoring set up at https://mms.10gen.com/host/detail/dcc39f13a93138e874baf774e498d6f. The increase in memory usage is after our most recently application deploy (on April 6) and a job on April 7 (around 12:00 PDT) to create a processed copy of all the files in GridFS, but I can't account for the increase in memory consumption that begins around 21:00 PDT on April 7 and has continued to a lesser degree.