It's very common...not a common "problem"...
Mine has it, and many others do to. Do a search in the archives for "catalina smile" and you'll read several ways to deal with it. I'll try to give you the short skinny right here: your keel is in no real danger of falling off (haven't heard of any documented cases of Catalina's shedding their keels); you can check the torque on the keel bolts, I can't remember off hand the torque value but I'm sure its in the archives too; and you can easily do a cosmetic repair to the hull-to-keel joint by opening the crack slightly with a grinder or dremmel and filling with epoxy thickened with high density filler to a peanut butter consistancy, fair and sand the crack and paint as required.And don't be surprised next time you have your boat hauled to see your Catalina smiling again. The crack often comes back.--Greg