If we are going to support N900 software, we need to support the dbus interfaces exposed by the N900 cellular services daemon.
The first post I am making is a list of the interfaces exposed by the cellular services daemon plugins and which closed-source binaries talk to those interfaces:

com.nokia.csd.Info: Used to obtain information from the cellular modem like the software version. Used by:
as-daemon-0 (daemon to synchronize email with Microsoft Exchange servers)
libmaesync (maesync library)

con.nokia.csd.Call: Handles phone calls. Used by:
libcodelockui1 (library for device code lock UI)
libconnui-cellular (common library for cellular connectivity UI)
libicd-network-gprs (internet connectivity daemon GPRS module)
librtcom-call-ui0 (library for dialer functionality)
rtcom-notification-ui (notification UI)
osso-systemui-devlock (systemui device lock plugin)
osso-systemui-emergency (systemui emergency plugin)
pulseaudio-module-nokia-voice (pulseaudio voice plugin)
telepathy-ring (telepathy module for cellular calls and SMS)

com.nokia.csd.GPRS: Handles cellular data. Used by:
libconnui (common library for connectivity UI)
libicd-network-gprs (internet connectivity daemon GPRS module)

com.nokia.csd.SS: Handles supplementary services. Used by:
libconnui-cellular (common library for cellular connectivity UI)
librtcom-call-ui0 (library for dialer functionality)

com.nokia.phone.net: Handles network related information. Used by:
clockd (userspace daemon for managing time and date)
gprs-provisioning (handles cellular data related provisioning)
nokia-maps-core (Nokia Maps core code)
libconnui-cellular (common library for cellular connectivity UI)
libicd-network-gprs (internet connectivity daemon GPRS module)
libosso-abook (addressbook UI library)
librtcom-call-ui0 (library for dialer functionality)
connui-conndlgs (connectivity dialogs daemon)
osso-startup-wizard (initial setup wizzard)

com.nokia.phone.sim: Handles SIM related information. Used by:
gprs-provisioning (handles cellular data related provisioning)
libconnui (common library for connectivity UI)
libconnui-cellular (common library for cellular connectivity UI)
osso-applet-device (control panel device plugin)
evolution-data-server-addressbook-backend-sim (backend for allowing address book to talk to SIM phonebook)
libicd-network-gprs (internet connectivity daemon GPRS module)
libosso-abook (addressbook UI library)
librtcom-call-ui0 (library for dialer functionality)
osso-startup-wizard (initial setup wizzard)
ota-settings (handles IAP settings sent over the network)
telepathy-ring (telepathy module for cellular calls and SMS)

com.nokia.phone.SSC: Handles cellular modem state. Used by:
libconnui-cellular (common library for cellular connectivity UI)
librtcom-call-ui0 (library for dialer functionality)
pulseaudio-module-nokia-voice (pulseaudio voice plugin)
osso-startup-wizard (initial setup wizzard)
telepathy-ring (telepathy module for cellular calls and SMS)
rtcom-messaging-ui (messaging UI)

com.nokia.phone.SMS: Handles SMS messaging. Used by:
telepathy-ring (telepathy module for cellular calls and SMS)
sms-manager (dispatches registered SMSs to where they need to go)