Most of the Olympus E-PL3's specifications rival or exceed those of other mirrorless cameras. It has the same sensor and processor as its more expensive sibling, the Olympus E-P3- plus a tiltable screen and 5.5 fps, both of which the E-P3 lacks. It is part of the only mirrorless camera line with in-body image stabilization, enabling IS for any legacy lenses. It is sure to be cheap- earlier iterations of the E-PL line have been known to dip into the $399 range and I expect it will start at around $599 for a kit. But I and many other interested photographers will not be considering a purchase of the E-PL3 when it is release this fall for one simple reason: no orientation sensor.
As one online commentator put it last year, when the E-PL1 was released without an orientation sensor: "No orientation sensor in 2010? How is that even possible?" Given the low cost of an orientation sensor on a camera that already has an internal stabilizer, this is clearly done merely out of a desire for Olympus to handicap its cheaper models- much as Panasonic has removed the microphone jack from its new G3's.
Olympus is not alone here- Panasonic doesn't include an orientation sensor with any of its mirrorless cameras (other than an external one on Mega OIS lenses). Olympus includes one in the $800ish E-P3.