About sending a message to a phone, it really depends on the provider, for example my company uses telus mobility for the company cells, telus provides a web page to send a message to any number on the network, so if I wrote a program that provided the correct post variables to the server then I could send a message from code, I think this is the sort of thing you would want to start looking at to see if it fits your needs.

In addtion to a website that is used to send SMS to cell phones, you can incoperate this into your prgram by simpily usng email. Most service providers assign email addresses to each indivdual phone this holds true for : Cingular, Sprint, Altel, Virizon Wirless, Nextel, Virgin Mobile, and most likely others.

heres an example of sending an sms to a TDMA network enabled phone: 0008675309@message.altel.net

heres an example of sending an sms to a GSM network enabled phone:
0008675309@msg.t-mobile.net

the 0008675309 is the phone number and what ever comes after the @ is service dependant so in your program you may have the user select their service provider and do some research for what the selected values should be (like altel's is message.altel.net)

have fun, not sure if this works in other contries outside the US

Hello!
You can send SMS messages via Java using SMProLib.
Please, take a look at http://mobiles.unipro.ru/smlib_eng.html
I have tried it and it works fine