This is a hoax which appears to be a rework of the MSN Petition Hoax, which was a rework of the AOL IM Charge Hoax and the Hotmail Charge Hoax. Can you see the pattern forming here?
Yahoo is going to charge for some of it's services, but there is NO evidence that Messenger is one of those services. The services that are likely to be charged for can be found here: http://biz.yahoo.com/fo/010730/0730dsf.html
Even if they did decide to charge for Messenger, do you really think they would take any notice of a Petition? NO......
Below is the one of the versions of this Hoax that is propagating via Chat and E-Mail:
Yahoo is planning to take away Yahoo Messenger by August 25th 2001. If you want to keep out Yahoo Messenger free of charge, send this to everyone you know. It will be used as a petition. Each person you this to counts as a "signature" If this petition gets 1000,000 signatures they will keep Yahoo Messener. If they do not receive 1000,000 votes you will have to pay $10.00 to have Messenger (per month). If you don't care about this then please for everyone.... copy and paste this message to everyone!