sbiddle: The problem is you can't easily do what you want to do. Knowing the location of cellsites isn't going to help you in any way at all and is completely and utterly irrevelent.
To know what phones are in an area you would need to query the HLR's of all 3 networks to establish the IMSI or IMEI's present within a geographic area, work out the numbers by cross referencing the IMSI or IMEI with the MSISDN to get the phone number and then send the SMS's. Quite simply, it's not going to happen without major negotiations with all 3 carriers and even then the changes that would be required to make changes to implement such a system are probably well beyond the sort of budget you have for such a project.
If you haven't already entered into negotiations with all 3 networks you should be doing this right now. You'll just be wasting your time otherwise.
I agree here with sbiddle, knowing cell ID's is not going to help, the operators will need to perform a HLR lookup to see what mobiles are connected to particular sites at that moment, then do the translation between IMEI and MSISDN, all of this taking up valuable resources within the network.
Unless someone was paying a lot of money or the government made it law for the operators to provide this service, I don't see the operators all agreeing and doing this for you.
Yes the Cell ID's will be of use in some sort of script, where you say to the operator, please generate a list of users connected to cellID's x,y & z and send them this message, however I'd say this instruction would have to come from the civil defence themselves if the operators agreed to do it.