YahooIM has an SMS to IM (and vice versa) command line interface that does not require any software, is always on as long as you stay signed in, and operates purely on SMS, which translates to no battery loss in standby. This will work on any phone regardless of OS.
Here are setup and usage instructions:
Signing in:
(1.1) Using text messaging, address a text (SMS) to:
92466
(1.2) Then, text:
in [Yahoo! ID] [password]. Do not include brackets.
(1.3) Your contacts will now see you as mobile.
To make it easier to sign in, you can save the information in (1.2) in the "My Text" option under the SMS menu on your phone. You may also want to add 92466 to your contact list as "YahooIM" so that it is easily recognizable and accessible. For an easier way to sign in, read the instructions under "Receiving and replying to a message" below.
Note: If you sign into the product from your phone, some carriers will send you an SMS that says you’re logged in. Sprint sends you a message telling you which users on your buddy list are online.
Receiving and replying to a message:
(1.1) Your Incoming Yahoo! Messenger for SMS messages will look like any other text message.
(1.2) Select
Reply and type your message.
(1.3) Click
send/OK.
Note: Each of your contacts will be assigned a unique SMS ID/phone number. After you've completed the signing in process the first time (which registers your phone) you can sign in without entering your username and password by simply sending one of your contacts a message. One way to skip entering your password for signing in is to add yourself to your contact list and just send yourself a message to login.
Initiating a message:
(1.1) Send an instant message to your Yahoo! Messenger contacts using their Yahoo! ID. To begin, address a text message (SMS) to:
92466
(1.2) Type:
to [Yahoo! ID] [message] Do not include brackets.
(1.3) Click
Send/OK.
Signing out:
(1.1) Using text messaging, address a text (SMS) to:
92466
(1.2) Then, text:
out
(1.3) Select
Send/OK.
Note: Some carriers will send you a text message confirmation that you are signed out of Yahoo! Messenger for SMS.
Pictures are available here if you need a visual aid to set this up:
http://mobile.yahoo.com/messenger/sms/signin-phone
Let me know if this has been helpful and if you have any questions.
Thanks!