The Z780 is a clamshell-style 3G phone designed to sell for a mass-market (cheap) price point, while still providing decent specs and features.
The key features include a QVGA display, HSDPA 3G data, 2 megapixel camera, A-GPS, stereo Bluetooth, and memory card slot. Some things that keep the price down: the camera is not auto-focus, and the external display is monochrome. Still, the feature set is decent and the thin profile is a nice bonus.
The monochrome display is worth a few words. The text is huge and highly visible. Like many monochrome displays, it's much easier to read in most conditions than a cheap color display (the type typically used on outer displays.) It's one-line, so long text has a scrolling banner effect (which is very smooth.) The time and most contact names (for caller-ID) will fit just fine without scrolling, though.
The Z780 has been spotted in FCC documents sporting an AT&T logo. A nearly identical phone - the TM506 - has also been spotted, sporting a T-Mobile logo. The TM506 appears to differ from the Z780 only in supporting the 1700 band that T-Mobile uses for 3G, instead of the 850/1900 that AT&T uses.
Bottom line: it looks like both T-Mobile and AT&T plan to carry a version of this phone. Sony Ericsson says they currently plan to ship the Z780 in the US by the end of Q3.
Here is a short video preview of the Z780: