Why are you so focused on Motorolas?
There are surely other brands that may be used.
As an example, I have 2 old SonyEricsson mobiles, that I can talk to over the serial interface.
My old SE t68i works fine, but only supports SMS in PDU mode, which makes it not so interesting to use.
On the other hand, my old SE z600 suppors SMS in text mode, making it very easy to use.
So far I have only tested sending SMS'es from the arduino --> phone, not tried to recieve anything.
Oh, and just to show off;
I have a free mobile plan. Costs nothing, and includes 50 SMS'es and 50 minutes per month, and costs nothing! 
pay to recieve a SMS... what a ripoff!