MongoDB does not reuse deleted space if the collection has an index on a key (in addition to _id)
there is already a pull request on github regaring that issue - but its not mine I just tried it.
Attached is a testscript(for 1.8.1) and the logs from v1.8.1 and the patched version(based on the v1.8 branch) from github. You can clearly see the difference.
Issue is also present on current master but not as bad(see the growth_test_master.log & growth_test_master_patched.log)