I like to use yahoo messenger and have been reading some of the posts here. Evidently Pre is not set up to use it yet, so I want to explain a work around.

Send a text message to 92466
In that message, type "in ID password" with no quotes and ID is your yahoo ID and password is your password.
Then you'll be logged in (you can also sign in from your PC)
You'll get a message in return saying you are online and you can type "help" for some instructions and there is a whole list of commands you can use.

To log out, type simply send a text to 92466 that says "out"

You'll get and send messages just as text messages to yahoo friends.