Get your Bento 3.0.3 update as soon as you can, it includes a big improvement:

Now, all iCal events with an End Date on or after 1/1/2008 will never be removed by Bento. In case you didn’t know, Bento had a four year window for events, based on the current date. Any event which was older than 2 years was automatically deleted from Bento (not from iCal). In case you had added custom information, that data was instantly gone for good, even from back-ups!

Good job, Ryan!