Since there is only this unhelpful no-no-no thread http://talk.maemo.org/showthread.php?t=48224 I feel bad since I am also an antennas toward the sky kind of guy. I will make one OT post here, otherwise it needs to be in another thread.

Satcom could be accomplished in the following ways:

Bluetooth link to a sat-SMS gadget like this has already been done with the N900 http://talk.maemo.org/showthread.php?t=86315 about $100 for the hackable puck and cheap enough monthly service for emergencies.

If you are a licensed radio amateur connect with an audio cable a yaggi antenna and a dual band walkie-talkie using fldigi on days that the ISS is doing packet radio BBS or slow scan TV downlink http://talk.maemo.org/showthread.php?t=82853 if you are just doing voice you only to the space station or the few remaining satellites you still need your N900 for tracking satellite overpasses, I use gpredict, thanks to Xes for compiling it for the N900 http://ge.tt/7mbX97r

Use your N900 with our very inexpensive software defined radio receiver, about $15 delivered for the host mode adapter and USB dongle http://talk.maemo.org/showthread.php?t=91182 with the right directional antenna you can listen to the amateur radio kids play with satellites or space station or grab weather satellite downlinks and decode them right in your phone, no transmit ability though.

Getting actual satellite telephony even if the N900 were a giant Motorola 1985 brick would be tricky, the frequency used is way out of band even if we had direct access to the radios inside the N900, the chunky spiral polarized antenna is half the size of the N900 and I think at least for Irridium the required hardware has not been made for years and new handsets are either from old stock or using either stockpiled Qualcom hardware or perhaps custom made chips, not an off the shelf solution AFAIK.
Below are the main players in the sat-phone game.
https://en.wikipedia.org/wiki/Iridium_satellite
https://en.wikipedia.org/wiki/Globalstar
https://en.wikipedia.org/wiki/Inmarsat