Regarding the alpha-6 installer:http://www.openplug.org/plugwiki/index.php/SheevaPlug_Installerhttp://plugcomputer.org/plugforum/index.php?topic=355.0
Make sure you read through the forum thread carefully to see what kinds of problems (and solutions) others have had. In particular, make sure you have good connectivity to the Console/JTAG:http://plugcomputer.org/plugforum/index.php?topic=454.0
and can access the material on the thumb drive ("usb info", "usb part", etc.) before attempting the upgrade. In particular, I had to explicitly reset the Plug before the upgrade to make it work. (But, even if it fails, it's not fatal: You can correct the problems and run the installer again.) What you'll wind up with is a newer Uboot and a 2.6.30-rc6 Linux kernel -- not the latest, but one that is much better than the default load.
Regarding x10, I've uploaded the current version of x10d.c that I'm using tohttp://cboh.org/SheevaPlug/x10d.c
Basically, if the program is invoked as "x10d", it assumes the role of a daemon server, connecting to the CM11a device, and listening for commands on a predefined port. If invoked as simply "x10", it becomes a client of x10d, sending commands to the server. I use cron in conjunctin with "x10" to control all my x10 devices. x10d also maintains a memory of which devices are on and which are off, and this can be queried, either to see the state, or create more complicated requests. It's pretty slick, and self-contained, but the downside is that I don't know of any good documentation for the software. I've never tried HeyU, but understand it is a fairly popular controller program, and I suspect that if x10d can talk to the CM11a via the PL2303, that HeyU won't have a problem, either.
Good luck, and let us know how everything turns out for you.