What are you trying to do? I'm working on a GPS Tracking Device (GTS) to recover stolen vehicles. I'm using an ATMega128 microcontroller and a MicroTek GSM cellular modem. Its pretty simple, just a serial connection between the two devices.
MicroTek sells the GSM modem and a development kit for $250. I didn't get any sample software code with it, but its 100% serial data, quite easy to program. It uses a DB9 connector that can connect directly up to a serial port on your computer.
The ATmega128 is probably overkill for what you need, but I needed both UARTs on it. An ATMega32 has a single UART and IIRC its a 16 Mhz controller with 32 KB program space. There are also tools to program it using C code, or Atmel provides an assembler you can use. The dev kit for this device was about $200 as well. I believe there are also schematics available so that you can make your own kits, and the processor will cost you about $4 or less.
Both kits come with HUGE documents, my modem has over 300 pages of commands, luckily I only need less than a dozen
It is capable of SMS though, and I have GSM service through T-Mobile.
Is this ANYTHING like what you wanted to know?