For those that read Engadget's initial review it said that the Galaxy Nexus does not support USB mass storage mode, but rather only MTP and PTP (media transfer protocols).
Check out this thread and quote about what that means and what to expect. First quote has a basic explanation of USB mass storage mode for your information.
Impromptu Q&A Session With Android Engineer Dan Morrill Brings To Light Reasons Behind Galaxy Nexus' Lack Of USB Mass Storage
Reddit thread: Whoa, whoa. ICS doesn't support USB mass storage? : Android
Check out this thread and quote about what that means and what to expect. First quote has a basic explanation of USB mass storage mode for your information.
ICS supports USB Mass Storage (UMS). The Galaxy Nexus does not. This is the same scenario as Honeycomb, as for instance HC supports USB Mass Storage while Xoom does not.
If a given device has a removable SD card it will support USB Mass Storage. If it has only built-in storage (like Xoom and Galaxy Nexus) it will (usually) support only MTP and PTP.
It isn't physically possible to support UMS on devices that don't have a dedicated partition for storage (like a removable SD card, or a separate partition like Nexus S.) This is because UMS is a block-level protocol that gives the host PC direct access to the physical blocks on the storage, so that Android cannot have it mounted at the same time.
With the unified storage model we introduced in Honeycomb, we share your full 32GB (or 16GB or whatever) between app data and media data. That is, no more staring sadly at your 5GB free on Nexus S when your internal app data partition has filled up -- it's all one big happy volume.
However the cost is that Android can no longer ever yield up the storage for the host PC to molest directly over USB. Instead we use MTP. On Windows (which the majority of users use), it has built-in MTP support in Explorer that makes it look exactly like a disk. On Linux and Mac it's sadly not as easy, but I have confidence that we'll see some work to make this better.
On the whole it's a much better experience on the phone.
Impromptu Q&A Session With Android Engineer Dan Morrill Brings To Light Reasons Behind Galaxy Nexus' Lack Of USB Mass Storage
PC-accessible storage is there, it's just not "USB mass storage" (by name). It's over MTP (Media Transfer Protocol). Basically the Galaxy Nexus, because there's no SD card, will let the /sdcard directory be accessible over USB.
As a user you will never see any of it. As a user, what you'll see is that it works exactly the same as before, except that you are using MTP instead of USB Mass Storage.
On Windows, this is pretty close to indistinguishable. On other platforms it's unfortunately a bit of a hoop to jump through at the moment. But that should hopefully change as people improve software like libmtp and mtpfuse.
Reddit thread: Whoa, whoa. ICS doesn't support USB mass storage? : Android