I was concerned about the storage of credit card info, too, but the current announcement of the feature on the LJ main page (http://www.livejournal.com/) proclaims:

As always, the safety of your personal information is a top priority. Rather than storing your payment data on our own servers, we encrypt the data and send it to Wells Fargo for secure storage.

More details are supposedly available in the FAQ, but I've yet to see any, so that's probably not very reassuring.