An arrest without a case definately means nothing. Not sure in Florida, but in Illinois.....
Once an offender is ARRESTED, you only have a certain number of hours to charge or release-usually 72 hours max. However, if the suspect comes in to be INTERVIEWED, which Zimmerman was, there is no time limit.
If the suspect gives an alibi, the police MUST break it, and that takes time. one way it can be done through locating witnesses, and having them come in to be intereviewed.
Evidence that was collected has to be worked up before any charges. That can take days, weeks or yes, even months.
THEN a State's Attorney (or District Attorney in C.S.I. speak) is brought in to review the case. It is the State's Attorney who then approves or rejects the charge for murder, not the arresting officer.
Once a person is arrested for murder, the suspect must go before a judge the following day for a bond hearing where the facts are presented to the judge. If there are still cracks in the investigation, the judge can release the suspect right then and there, so naturally the state wants the strongest case possible BEFORE that happens.
Everyone needs to take a deep breath with this case. I say bring in some assistance for the investigating department in case they do not have much experience with homicides.
Zimmerman will be charged with something, it's going to just take time.