Personally, I think that "In math we trust" would be better than "In cryptography we trust", since more people would understand that.
However, I think the whole "In xxx we trust" is a bad idea from a PR perspective, since whatever the xxx, it is obviously replacing "God". That will leave a bad taste in the mouths of very many people. Many fundamentalists would probably end up swearing off bitcoin for no other reason.
There was actually a discussion about creating a bitcoin motto a while back in a very similar thread to this one:
http://forum.bitcoin.org/index.php?topic=6455.20My favorite suggestion from that thread is:
Iunctum in Divortium (Unity in Division)