Mobile Audio/Modem Daughter Card
The mobile audio/modem daughter card (MDC) specification provides a mobile form factor and interface for audio and modem solutions based upon AC'97. The MDC will liberate modem board manufacturers from the burden of repeated certification of modem cards tailored for each notebook model. I believe that only the modem section of this specification is used inside the notebooks, which is why I only introduce the modem here.
Such a MDC Modem is quite easy to design and produce. The modem drivers are part of the audio driver.
An example of such a MDC card is taken from an ASUS L8400B Notebook.
MDC as well as MiniPCI do not support aftermarket upgrades to the sealed notebook, but I think it is still worth knowing about them.