Thanks for posting!
I was able to reproduce this issue between Bento 4 and FileMaker Pro 11 and preliminary testing points to multiple versions of Bento being installed as the culprit. I've forwarded this post to our Development and Quality Assurance (testing) departments for further investigation.
Just purchased today (last day of half price sale) and I am getting the exact same message. When I simply choose the file type of "Bento" I get this error and it actually locks up the app (FM Pro). Would love to actually use this software I just paid for.
Just wanted to add my resolution. Re-reading the comment from TSBear, I realized that I had both Bento 4 AND Bento 3 in my applications folder. I dragged version 3 to the trash and all was well.