I saw those on Wednesday actually (beforethe PDD birthday bash).  So does the city take them down when the currents "go away" or are there just always rip currents there?  It's something I grew up with so I have never thought about it as something that needs a flag.  But hey, I'm all for safety.

The firefighter at Station Five checks the surf zone forecast online each day from June 15th to October 15th and flys either a green, yellow, or red flag at the locations mentioned by Paul.  Affixed to each flag pole is a rip current awareness sign, as well as a sign explaining the beach flag warning system:  

Green Flag- Used to indicate calm conditions. Signs should be closed indicating beaches open.

Yellow Flag- Used to indicate moderate surf, possible rip currents and/or deteriorating conditions. Signs should be open indicating beaches closed.

Red Flag- Conditions indicate strong rip currents. All persons advised to stay out of water. Signs should be open indicating beaches closed.

If you're ever caught in a rip current remain calm so you can think clearly.  Don't fight the current, instead swim parallel to the beach until out of the current and then swim diagnonally to shore.