low+cost+gprs+module
One thing to remember about this is that any end design that has a cellular modem in it will need to be PTCRB certified, which can cost $20,000+.
There is a product from Janus that is certified as an end product, but can be developed on just like a GSM module: **broken link removed**
It is mainly used for proof of concept, but I've seen many engineers go with this solution long term. You can always spin a board later, when you are ready tos hell out the cash for certification.
-B