Error starting MongoDB on CentOS
In my case this turned out to be caused by not enough memory

I concluded this after running some of the commands found in this article: 
http://www.cyberciti.biz/faq/linux-check-memory-usage/

egrep --color 'Mem|Cache|Swap' /proc/meminfo will output how much memory is available. Don't forget that Mongo itself will require memory, so even if you have got a bit of free mem, it may not be enough.

I restarted my server, which killed a few of the memory sucking processes, and this for now has solved my problem.