You can send SMS messages to mobile phones using iChat, iChat AV, AIM messenger (AIM4.6b1.1131 and later releases) and probably by other compatible messengers using AIM network (Fire, Proteus, Gaim etc. I have not tested these, only iChat, iChat av and Gaim).
In iChat, go to File > New Chat with Person (or use the shortcut, apple-shift-n). In the window, put as the screenname the mobile phone number. If the number is e.g. a US (123)-456-7890, type +11234567890 as the person name. You have to use the number without "()" and "-"s in the middle, and in the international format, thus +1 is equivalent to US. The mobile phone has to be sms compatible; most GSM mobile phones are that. This works to the (or most) US mobile phones; AIM probably does not support this to other countries yet.
After adding the number, hit enter. Type the message. Hit enter again. You will get a message saying "Your IM has been sent to my cell phone and, although I can read your messages, I cannot reply. Thanks for your IM! Want your IMs forwarded to your cell phone? Click here.". The message will show your AIM handle as sender.
In iChat, go to File > New Chat with Person (or use the shortcut, apple-shift-n). In the window, put as the screenname the mobile phone number. If the number is e.g. a US (123)-456-7890, type +11234567890 as the person name. You have to use the number without "()" and "-"s in the middle, and in the international format, thus +1 is equivalent to US. The mobile phone has to be sms compatible; most GSM mobile phones are that. This works to the (or most) US mobile phones; AIM probably does not support this to other countries yet.
After adding the number, hit enter. Type the message. Hit enter again. You will get a message saying "Your IM has been sent to my cell phone and, although I can read your messages, I cannot reply. Thanks for your IM! Want your IMs forwarded to your cell phone? Click here.". The message will show your AIM handle as sender.