Google updated the "My Account" page today with a detailed breakdown of the storage you are using -- both free and paid. For example, you can see how much of your unpaid Gmail and Picasa Web Album storage is being used. In addition to this information, you can also see your paid storage usage with the option of adding more. To upgrade, you can choose which package you want and click the Google Checkout button.
Unlimited storage, especially if they are toying with the idea of file backup using GDrive, could be too costly for them. So maybe they are getting people used to the whole "pay-for-storage" model. This could have a couple benefits. First, they will make money, and second, they should drive away some of the people who would normally abuse free storage for illegal purposes.
Even if they do offer some storage with the base GDrive offering, it will likely be a small amount like 1 to 5GB -- certainly not unlimited.
What do you think? Will GDrive be backed on a pay-for-storage model? Or will it be free, unlimited storage?