It seems there is a small bug, or issue in Facebook Timeline that does not allow you to delete certain Recent Activity entries. If you create a new Life Event (maybe other type events, also) and give it a date of sometime more than 1 month ago, then, try to delete it's "Recent Activity" entry, you can't. You can delete all other types of Recent Activity, but not a Life Event Recent Activity (I am referring to the Recent Activity entry only, not the actual Life Event). When you delete it, it goes away (deletes), but as soon as you return to your page, it's right there again.
I've confirmed it on two Facebook accounts - can anyone else with Timeline activated delete these Recent Activities? Maybe i'm just doing it wrong :-/ .. Still a few weeks to go before Timeline gets forced on everyone, I'm sure they'll get it worked out.