Due to the reduced resistance imposed by LEDs, you may experience a "fast blinker", similar to the effect of a burned out bulb. This is because the turn signal timer circuit is charging too quickly due to the decreased load. SuperBrightLEDs.com sells a replacement flasher module, part number CF13GL-02. This can be purchased at their car parts page; just look for the LED Bulb Flasher Unit and select CF13GL-02 from the popup list to buy it. This replaces the flasher module mounted under the dashboard near the steering column, and should restore normal turn signal operation.
Alternatively, you can add an appropriately-sized resistor to the circuit, but this is probably more trouble than just replacing the flasher module.