chan_celliax

Celliax is a GPL channel driver for Asterisk, chan_celliax, development and download site http://www.celliax.org

runs on Windows and Linux
manages GSM and CDMA cellular phones through an adapter, composed by a serial datacable (for commands) and an audiocable (for the voice) interfacing the computer soundcard
Celliax is also capable of making and receiving Skype calls through the cellphone, and has an app like app_directory that let you choose which one of your Skype contacts you want to call.
on Linux Celliax can also manage ALSA voicemodems (whithout Skype support)
on Windows and Linux (and *BSD) manages virtually all kind of cellular phones, both old Nokia models (with proprietary Nokia FBUS2 commands) and cellphones supporting AT commands (also with Motorola extensions)
with Celliax is distributed the Celliax LiveCD, with a working installation of Asterisk, chan_celliax, and configuration utilities, based on Knoppix. The Celliax LiveCD contains also all is needed to run Celliax on Windows: the cygwin installer and the tgz with the asterisk-celliax stuff to be untarred in a basic cygwin installation.
Headless Celliax, cellphones, Asterisk and Skype (without X)! Booting the LiveCD in console mode you find a script (/usr/local/asterisk/usr/sbin/celliax_server) that automatically launch Xvfb (the fake X11 server), Skype and Celliax. Also via ssh! Isn't that cute?

Celliax development and download site http://www.celliax.org

Example: Siemens mobile phone

German forum: Build your own cable for chan_celliax

Some notes on connecting asterisk to the cellular network. Why?

Leverage free mobile to mobile calling to lower costs. (nice picture)
SMS gateway (See note). An easy way to send SMS messages to mobiles from Asterisk is to use the FASTSMS command or the SMSx command.
Mobile applications; geo-location tracking or theft prevention for example
Remote monitoring
Mobile Machine to Mobile Machine so called M2M

The cellular capable device you attach might be.

Interesting notes in the asterisk mailing lists.

Vendor

ICS France offers Top-GSM series Fixed Wireless Access Terminals, 1, 4, 8 Channels.
http://www.sipcpe.com/fx300GSM.html - SIPCPE offers the FX-300 GSM Call Director a GSM <--> VOIP/PSTN switch and bridge
CellSocket - Converts cell phone to a regular line phone.
CellDock - same thing as CellSocket, but supports multiple brands on same base through converter module.
See eBay
Ericsson Fixed Wireless Access Terminals
http://www.falcom.de/e/e-txt-produkte-industry0.php?auswahl=
Falcom range of stuff include Global Position System.
2N Telekomunikace Single-line GSM/FXS/FXO and Multi-line GSM/VoIP Gateways. It uses Siemens GSM modules.
PhoneLabs - Converts cell phone a regular phone line.
- Supports 400 cell phone models
- Bluetooth capable
Vox Link or VoxLink Cellular Docking station; similar to CellSocket. You can find these for sale, but the vendor seems to have disappeared. (Went out of business)
Siemens - see: Asterisk Siemens M20
Telular - large range of "Fixed Wireless" products.
Eurotech - Makers of Analog Digital and VOIP to GSM/CDMA gateways. from 1 channel to 32 channels 1-4 sims per channel for advanced LCR. Works seamlessly with Asterisk.

Details regarding Serria Aircard as possible bridge.

PCI Cards with a GSM interface

Voismart - Voismart vGSM with up to 4 GSM channels PCI card, made for Asterisk and linux in general. See also VoiSmartGSM for instructions and GPL drivers
Junghanns - uno/duo/quad GSM PCI. Works with Linux