I know when we first started planning, my Wedding Planning for Dummies book said that for Catholic weddings it's common to not be allowed to 'bare shoulders' and that things should have sleeves. As you know, my dress is strapless.

It seems to depend on the specific church. The Deacon who is doing our ceremony seemed surprised when I made sure my dress was appropriate, so obviously that's not a very big deal in our church. On the other hand, I often wear jeans to Mass... so it wasn't a big shocker to me that there wasn't some strict dress code. If I were you I would check with the bride, but I imagine a sleeveless dress is just fine. If you're worried about it, you could throw on a cute cardigan or something for the ceremony, but I imagine if it were an issue the bride would have told you when she asked you to do a reading. I mean, what would they do, kick out people who come to the ceremony who aren't dressed with their shoulders covered? haha.
Check with the bride, but I bet your dress will be just fine!
(and, as if I even had to ask... POST PICS!)