C.M.
I had a Mirena from the time my son was 6 wks old until he was about 2 1/2 yrs. I had it taken out in June, bled for about 10 days, then was able to get pregnant on July 1st (2005, not 4 days ago). It was really interesting to calculate my ovulation times, basal body temp, etc. It wasn't that I had any difficulty getting pregnant either time, I just wanted to be more educated about how my body works (especially after having the IUD and not knowing what to expect).
And, I had 2 totally opposite pregnancies, so they are all different.
Good Luck.
And - for the record, I loved my Mirena and would get another, I just haven't gotten around to it, EVEN THOUGH my husband has had a vasectomy and we have no risk of getting pregnant(this is husband #2, BTW, and he had his snip done 20 yrs ago - so no talk about failed vasectomies, etc.). It was really nice to have the other benefits of the Mirena - no period, etc.