Funny thing is that both Arduino MEGA and GSM_Shield library are "supported" by the IComSat.(At least it's said in its website :p)
Not IcomSat - they only make the SIM900 chip, IteadStudio is responsible for the library and the shield it is mounted on. FWIW, I found the minimal libraries to be OK for SMS and like functionality but totally inadequate for GPRS operation. For that, I ended up writing my own series of commands to make the SIM900 module do exactly what I wanted it to (i.e. transmit to Cosm).
I use iTeadstudio quite a bit for PCB manufacturing but "support" in general for their goods is less than optimal. For example, not all devices they offer come with drawings or libraries, making it difficult to incorporate them into your designs. You get a nice apology note if you point it out to them, but I have yet to notice changes to their web site in response to customer inquiries.
Seeed Studio seems related to itead but separate enough to offer better support. If you want really good support, buy from Adafruit Industries. They publish great tutorials for almost all the stuff they sell, are responsive to tech support inquiries, and offer helpful forums also. You pay extra for that support but them's the rules...