Reproduce using java program : http://pastie.org/1428863
Change CONNECTION_COUNT to 128 and the problem disappears, 129 or higher and the problem occurs 100% of the time on fast machines. Slower machines tend to not reach the 128 current lock point.
db.serverStatus().globalLock.currentQueue.writers will show 128 and not recover, regardless of load or wait time. Verify by doing db.test.getIndexes(), it never returns after this.
mongostats output after test to confirm load is in fact gone : http://pastie.org/1428842
Other hosts can successfully connect to the same mongo instance.