It seems that many of my assumptions about a Bluetooth driver for the Newton were correct and it is indeed possible to implement such a thing. The basic idea is to implement a new comm tool based on the TAsyncSerTool which would provide the RFCOMM layer. This is very similar to the work I've done on Nitro, it might even be a bit cleaner ... and run on on older MessagePads as well.
One common low-level interface to Bluetooth cards is a UART. This means that we can simply talk to the card via a serial channel. There are different kinds of UARTs - if a card would use a UART the built-in TSerialChip driver won't recognize, we would have to implement a new TSerialChip driver. But that is fortunately not required, the Newton seems to recognize most UARTs just fine.
The next layer is the Bluetooth HCI layer. It is a set of commands and events encapsulated in data packets. It provides functions to establish connections, do the key and PIN code negotiations, transmit data and so forth.
On top of the HCI layer goes the L2CAP layer. It provides logical connections over a physcal HCI connection. This will be the basis for the RFCOMM layer.
... let the fun begin! The card is a PICO PCMCIA Type II Bluetooth card. Currently, there are large numbers being sold on eBay in Germany, so I invested the donations I've received so far (thanks guys!) and got one for myself.
This means that I can start with the probably insane task of implementing a Bluetooth stack for the Newton. At least the RFCOMM part...
As for the cards that finally might be supported: They have to be PCMCIA Type II cards (16 bit, 5 Volt - 3.3 Volt might work too). CardBus cards such as the newer Belkin models won't work.
The foundations of a speech regcognition codec are now more or less done, they're on the Experiments page. Silence detection and input signal quality are probably the biggest problems so far, but the filtering part looks good.
I'm not sure if it is useful for anybody, but I just uploaded my old serial test code and the modified Endpoint code. It's under Various Experiments.