Yes, they may have to go for an indictment FIRST -- it does appear they simply do not have enough physical evidence to arrest Casey. If they did, they would have arrested her by now -- I would think.
They may be looking for that one big ticket break -- some huge piece of evidence -- like a body, which they haven't found yet. Of course, you don't need a dead body to try a murder case, but still, there does seem to be a stopgap with their inability to pinpoint this on Casey.
Perhaps they are compiling evidence, slow as it is, which may indicate another person's involvement -- could be more than two people besides Casey involved here.
I viewed all of Cindy Anthony's interview -- as far as I can tell, she clearly does not know what happened to Casey. She did admit that if she were not connected to Casey and was viewing this case from an bystander's point of view, her view of Casey would be extremely negative.
She's obviously not wanting to look at the possibility that Casey had anything at all to do with Caylee's disappearance.
Casey may indeed, walk -- unless they can find enough evidence to take her to trial.
That may not happen -- unless Casey's body washes up on shore or someone finds some piece of solid evidence tying Casey to this disappearance. Thus far, all they've seemed to find to arrest Casey is for check kiting --