Credit:
Mraz's account is currently sending out links to a Cost Per Action offers site that appears to add $9.99 to your phone bill if you follow through with the request. There's some social engineering going on here too, with the Tweets reading: "Here is the quiz. whoever beats my IQ of 97 will win the ipad and get to be in my newest video".
The hack was confirmed on Mraz's Facebook page minutes ago, with an update reading: "**IMPORTANT** someone has hacked into Jason's twitter account. Don't believe anything that is tweeted until we have access back into the account!"
Hat-tip: Arthur Baynes