It's definitely possible, but doesn't work right away, it requires some setup:
First, you need a Windows Live Account (actually any compatible Online ID service would work, but so far, there are no others). They are available for free, and you get a whole bunch of useful functionality (online harddisk, Live Mesh, which was already mentiones, synchronised calendar and contacts for multiple PCs and more).
The, you need to link this account to one user profile on all PCs that you want to be able to participate in the streaming network. This can be done in the usual User account screen (where you can change your password and everything). On the left side, you should see an option that has "Online ID" in it (I'm using the German version, so I'm not sure how it's called in English).
There you'll find a screen asking you for your Windows Live ID and password.
Once you entered these, your user profiles are linked through this account and identifyable online.
Then you open Windows Media Player, open the "streaming" menu and select "allow internet media streaming" (or something similar, as I said, German version here).
Once you select this option, your PC should send you through a series of dialogues, some of which require admin access. During this rather bureaucratic time, your Firewall is set to allow for the streaming access and your Library is set as shared.
Then, when you open WMP on one of the connected PCs, a new remote library should appear (unless you disabled this function). This should be named after your user profile and Live ID.
In this, you can find everything that's in your library, from any place with an internet connection.
I'm sure this works, since I used this funtion to watch movvies with my girlfriend at her home, while the files were on my PC in my dorm room.
Once your account is linked to Windows Live, you can also select this account in File Sharing. This does not work online, though, in this case the account is just used to identify the authorised user.
I think if you share your Media Library to a Windows Live ID you know, the ID owner will also be able to access it online, but I haven't tried that yet, since I don't know many Windows Live members.
If you do this, you should probably also install Windows Live Essentials, at least Live Mesh, since even if you don't have a second PC, it offers an automated online backup of any files you select. If you have a second PC, you can keep several folders in sync, and just continue work at home which you started on your notebook at work. That's probably the one Windows Live component I use the most, besides the calendar and contacts synchronisation via Outlook.
It will even keep your IE favourites, and offers Remote Desktop functionality.
