It is now possible to send text messages to people's cell phones right from your Gmail chat once you enter the persons number. Google tried this a little while back, but had some technical issues. All the bugs seem to have been ironed out and it is operational again.
Your friends can reply to the text and the message will make its way back to your chat box. The messages will count as normal text messages, so you might want to check to make sure you are not costing your friends a small fortune. The recipient does have the option to block individual senders or everyone at the same time by replying with the words "Block" or "Stop" respectively.
"A few weeks back, we ran into a few snags when we first started rolling this out, but starting today you can turn on text messaging for chat. Just click on Settings, and go to the Labs tab. Scroll down until you see "Text Messaging (SMS) in Chat" and select Enable and Save Changes.
We'll save your friends' numbers in your Contacts, so next time you can just type their name in the chat box and select Send SMS.
On the receiving end, when you get a text message from Gmail on your phone, it will come from a number in the 406 area code. (The l33t folks in the crowd will note that this spells G0O.) You can reply to this text on your phone just like you'd reply to any other text. The reply gets routed back to our Gmail servers and shows up in your friend's Gmail chat window. Each of your friends' messages will come from a different 406 number so you can reply to any message and it will get back to the right person. Messages from the same person will always come from the same number, so you can even bookmark it in your phone.
If you get a message from somebody you don't want to chat with from your phone, just reply with the word BLOCK. If you don't want to get texts from anybody using Gmail, reply with the word STOP and we'll leave you alone. Keep in mind that all these text messages count as part of your regular mobile messaging plan and might incur fees. So unless you know your friends have unlimited text message plans, please be sensitive to their phone bills."