On this page, Palm has started to release source code + patches for a number of FOSS programs that they use in the Pre. I suppose the page is only an interim solution, since the entire site (nor the page URL) doesn't yet really seem to consider the fact of OS updates, etc.
Of course I have no idea yet if those sources can be considered complete and corresponding, but at least an initial look seems quite promising.
I've spent about 10 minutes looking at their 9 MByte (!) kernel patch against vanilla 2.6.24. The modem interface seems to be a UART + USB. The UART is required for stuff like waking up the OMAP3 from the baseband, and then you use it to set up a USB connection to the modem, where a hacked/extended version of the cdc-acm driver appears to be used.
I don't have time to look further into it, but I'm sure somebody with actual device hardware will - now that the source is out there.