Android 4.0 Supports Mass Storage, But Galaxy Nexus Won't
According to Google, Android 4.0 Ice Cream Sandwich does support USB Mass Storage mode for external storage, but not for internal memory. The reason for this is because all systems running Android 3.0 and up unify how they see internal memory (meaning no longer separate memory for media storage and for applications). The code for Android 3.0/4.0 uses a protocol called MTP to manage internal memory. Windows-based PCs can recognize MTP and see it as a separate drive, but Apple computers do not. This means any device that has only internal memory for apps and media — such as the Samsung Galaxy Nexus — won't support USB Mass Storage. Devices that have external memory cards (i.e., microSD cards) will still be able to support USB Mass Storage. Using the new unified view of memory allows users to access their full storage allotment for applications while keeping the operating system safe. The trade-off is that many devices will no longer have traditional USB Mass Storage support. Instead, Android users will have to rely on a third-party syncing client that supports the MTP protocol to gain direct access to their device storage.
Honestly, why would I want such a phone that is hamstringed out of the gate? For what, ICS? Not really worth it to me.