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The Zaurus SL-C860 runs with Debian Linux, and I would say it's the best real pocket PC (original sense) you can get:

http://www.dynamism.com/zaurus/index.shtml
http://pocketworkstation.org/

Let's look at your list and compare:

*400 MHz Intel XScale CPU

(+) YES

* 128 MB (but faster) RAM

(+/-) 192 MB RAM, but 128MB are flash ram

* 1MP color still camera & 320x240 DV camcorder/web-camera (with recording capabilities)

(-) It doesn't have a camera.

* Bluetooth and Infrared capabilities (to synchronize my... phone

(+/-) The SD slot doesn't support SDIO, so you must find a CF-Card with Bluetooth AND Wifi and it must be compatible with Debian. I don't think such a card is currently available. Infrared is available onboard.

* Good Wi-Fi support (this is an absolute must)

(+) You need a Wifi-CF-Card. They work great with Debian. (add $79)

* 8 MB ATi Radeon with hardware 3D capabilities (poor man's gaming console)

(-) no 3D-acceleration

* 16bit sound (good quality speaker, earphone jack, built-in Microphone (Line-In?))

(+/-) 16-bit sound, speaker not too bad, earphone-jack, microphone not included, but you can use the earphone-jack as a microphone-jack.

* Jog Wheel (to scroll web pages!)

(-) NO

* Handwriting recognition, basic voice command/recognition, Voice-recorder capabilities

(+/-) You can use all the Debian packages, so you will find voice-recorder software and handwriting recognition, but the handwriting recognition will be very bad compared to a PocketPC or an old Newton 2000.

* Upgradable OS feature with good such support from manufacturer

(+/-) Yes, you can always upgrade Debian to the latest version, but there is no support from the Manufacturer.

* 1-2 built-in expansion slots (whatever's best, might be an idea to include a real hard drive)

(+) SD- and CF-slot, but you will need the CF-slot for your WiFi-card.

* 2 USB ports (including support for USB key external drive)

(-) no. Its USB port is only used as a network port for connecting to a PC (via TCP/IP)

* 24bit color VGA 640x480 backlight TFT (or 320x320 in worst case)

(+/-) The Zaurus has definitively the best screen that's currently available in ANY PDA. If you've ever seen it, you will understand. Nevertheless, it doesn't have more than 64000 colors at 640*480.

* Good software/hardware Mp3/Ogg capabilities (poor man's iPod)

(+) Yes, no problem using standard debian software. However, as those (X11-)applications are made for use with a mouse, you'll probably need your pen to operate them.

* Reasonable battery life (I realize that all these features will drive battery life down)

(+) When you use Wifi, you'll get about 3 hours, without Wifi up to 8 hours.

* Up to 300 grams and with size similar to today's PocketPCs

(+) 4.25" x 3.25" x 0.8", 250 grams

* Bags of software and some shareware apps/games pre-installed

(-) No, the unit comes with Qtopia, but because you want a "pocket workstation", you will want to use Debian, which is not pre-installed.

* [If phone capabilities are doable, that's good. If not, that's ok too, for now]

(-) No, you cannot use it as a phone.

All that for no more than $599 US.

Zaurus+Wifi-CF-Card: $928 US and while it really has a lot of features, it's not at all: "All that". So maybe you understand why I called your $599 expectation absolutely crazy ;-)