Is the stylus of yesteryear PDAs staging a comeback? InPlay Technologies has introduced its WriteSense digital pen technology, which combines active RF pen input with the capacitive touch-screen capabilities of the iPhone and G1.

Capacitive touch screens do not have a digitizer layer for analog input, which makes them useful for fingers—as they require a finger’s ability to provide capacitive coupling to interact with the screen—but not so much for the type of stylus Palm first made famous for its PDAs and smartphones. You can’t even use a fingernail with a capacitive display.

WriteSense is a solution that offers high-resolution pen capability and gesture support through digital stream communication. It's also low-power and battery-free.

According to ABI Research, revenue from the global touch-screen market for mobile phones and other handheld devices will hit $5 billion in 2009.

“Our new technology enables active pen capability at an incremental cost when capacitive touch systems are used,” said InPlay president and CEO Van Potter in a statement. “We believe that our technology could be rapidly adopted in the tablet PC and e-reader markets. We are also seeing strong interest from the smartphone segment.”

WriteSense is slated to appear in smartphones by the middle of the coming year.