USB Control of WS2801 LEDs
You do need to have common grounds between the strip and the microcontroller module.

The WS2801 PWM driver reads 24 bits of data and passes the rest on to the next module in series. It latches when the signal is low for >500&mu;s.

If you're getting only one pixel to light, then the time between each set of 24 bits is too long, and the first pixel is treating each set as its own.

Datasheet for WS2801.