After about 10 days of research and visiting shops, I purchased the NTT DoCoMo FOMA P2402 card for my C3000. It is a 384K downstream top speed service and it iis essentially plug n' play -- no need to download any software or drivers. Their slower 64k "P-in Free" (who names these things?) also should work.
It took an hour from start to finish to work my way through the Q&A process and get them to make the sale of the P2402 (in stock in Gotanda) and walk out with a functioning card. There is a lot of car salesman stuff going on in the DoCoMo shops trying to get customers to go for extended plans and all sorts of options so my caveat is to study the plans carefully before signing on the dotted line. DoCoMo also assigns the CF card a regular keitai phone number although that does not turn your PDA into a cell phone.
My regular keitai is KDDI au and they were my first stop. However, after being told at one au shop that their 2.4 Mbps Packet-Win service would work with the C3000, I was informed when I went to another au shop to sign up that this is not the case. Zannen.
My experience at Vodafone was the worst. At their Kami Ozaki shop in Tokyo the young women there basically did not want to deal with my questions (I am a gaijin, I speak fluent Japanese and brush my teeth three times a day.) They had no idea about the VC701si card (384k) until I pointed it out to them in their catalogue. They basically declined at first to find out for me if it would work with the Zaurus. They suggested I call Sharp. When I suggested they get THEIR customer or tech support on the phone and try to make a sale, they reluctantly agreed. Customer support was equally clueless and tech support thought it might work but wasn't sure. If anyone runs this gauntlet and gets a more precise answer, please let us know.
There were no cards in stock at either the Kami Ozaki and Gotanda Vodafone shops and no one could tell me how long it would take to order, so I basically gave up on Vodafone. I like to support the underdog and the foreign entrant -- all things else being equal -- but compared to DoCoMo (which runs their busy shops like a very efficient bank) Vodaphone seems solely focused on selling cute cell phones to the teen crowd.
DDI Pocket also has a number of CF cards which link to their PHS service. But I would only recommend looking at these if one finds 64k a sufficient speed and does not plan on using the PDA in motion.