I've found the ROKR E2, which is yet another Motorola Linux GSM/GPRS phone exclusively sold in china so far. Apparently since June 22nd, so it's a quite new thing. It's very different from the A7xx/E680x series in that it doesn't have a touch screen, but many more buttons. Also, it features a full-size SD card slot, which makes it theoretically SDIO compatible (I'm pretty sure they use some SDIO compatible SD host controller in there).
Let's see whether I can work with the Chinese language firmware. I already found out how to get it into boot-loader flash mode (by pressing the camera button on the upper right side while powering the device up). It looks completely different than the blob on the A780/E680, but that doesn't really mean anything.
As of now, I don't have any technical proof that the device runs Linux. I'll probably not find time to play with this toy before I get back to Germany. But if anyone has hints or further information on how to dig deeper into the ROKR E2, don't hesitate to send me an email about your findings.