Hi there, guys.
I'm new here, just found out about RasPi. I'm not a developer, but as a general geek and "light" DIY modder, this is actually something that crossed my mind (along many other things... I can actually see myself building a few of these at home, ranging from a simple HTPC to a POS for my brother's gym), especially since I'm very much into NAS.
Please let me chime in on a couple of things I've learned over the years. I won't touch the whole "RAID is not for backup" issue, it has been discussed already, but there are a few bits of information you might find interesting.
First up, and from what I've read so far, as for NAS devices go, just about every embedded CPU available over the last few years can handle a 10/100 Ethernet connection just fine, and max it out. Most of them can also read and write to an HDD at about the same speed (which is pretty much all you can expect to get from a RasPi, since even 450Mbps WiFi doesn't usually get much higher than Ethernet when you start moving away). In short, a Raspi should be just fine for a NAS, 30MBps total throughput seems very doable even if every interface is running over USB.
Next, someone asked about RAID5 and USB. Well, I have seen SATA/USB controllers that claim RAID capabilities (not sure if they are software RAID, though, which would probably cripple the Raspi HDD performance tremendly). They are so-called "smart" Port Multipliers. Please take a look at the Addonics AD4SR5HPMUS: USB or SATA uplink, 4xSATA HDD connections, several choices of RAID available.
Now, if what you really want is data redundancy without fussing with RAID, a.k.a. Drive Extender style, you can actually have that in Linux. Look up Greyhole, which was created to be a Linux Drive Extender. The Amahi NAS distro already incorporates it, and AFAIK is freely available and is built on top of Samba, so it should be a breeze to make available on a RasPi device, right?
Btw, I'm not a Linux guy. I've tried it, more than once, actually, but I've always fallen back to Windows (too many years being a Windows power user, I guess...), I just can't seem to get Linux to work the way I want it to, like I do with Windows (I know what you're thinking, but I'm not even remotely inclined to programming/math in general, and too many terminal commands make my head spin).
Anyway, one last thing... Am I the only one thinking with a Model B, some extra circuitry, an HDD and an enclosure, one could make a cheaper plug-able computer, WITH storage included? Or a much cheaper "just-add-your-own-HDD" version? I mean, it could even be powered directly from the socket...