According to Inside Mobile Apps website, Microsoft will phase out its proprietary "Microsoft Points" from Xbox LIVE and replace it by real world currencies.
We still don't have confirmations from other sources but - as far as we know - Inside Mobile Apps is a reputable web site.
The website claims that it has received word from several developers who have been notified about the eminent change and told to plan their future game updates accordingly.
Microsoft Points are currently being used on Xbox LIVE, the Zune Marketplace and the Windows Phone Market but only Xbox LIVE doesn't have an option to pay in real world currencies. It sounds more reasonable to us that that Microsoft would allow paying in real world currencies on Xbox LIVE without removing the option to pay in Microsoft Points.
As usual, Microsoft refused to "comment on rumors or speculation."