add ZFS RAIDZ, it's going to cost enough anyway
I can't really see it being a SAN/NAS/RAID media box, personally.
though, a mac mini with DVI + HDMI + a lacie 5big type setup in a single case/cube, and perhaps a blu-ray drive for 1080p content, it's nice. comfortable. i'd buy one. though it would cost 2x more than a PS3. and really, do a lot less.
it's likely more desirable than a PS3 too. in serious comparison though, the wide range of colourful and feature-packed RAID5 boxes are a dime a dozen (give or take 3 thousand to 7 thousand dimes), and arguably, HP's media server would be remarkably hard to improve upon without a prolific cost increase.
though apple's pricing usually makes sane people cringe, a $1500+/600e+ time capsule i.e. a multi-terabyte appleTV + time capsule hybrid would be futile and catastrophic for apple, (though very handy for XBMC/boxee users ultimately)
mostly due to the fact that people don't like spending $1000+ on what's essentially a PVR that doesn't record. it would really have to do a lot more than appleTV or time capsule. a lot more.
my notion is, since it's likely to come out around the time of Snow Leopard, the addition of a cutdown revision of OSX server using ZFS for RAID0/1/5/6 with a classically brilliantly simple GUI interface. since ZFS includes automatic snapshots, automatic integrity checks, radically different journalling & rebuilds to checksum & protect data. apart from the gushing nature of ZFS, it's got actual data integrity checks which should increase the safety or useful life of a multi-hdd system exponentially.
additionally, and more importantly, the inclusion of openCL to accelerate encoding x264 video would indeed be pretty handy on a media box for transcoding/content shifting.
as 1080p x264 decode would be a functional requirement of any post-2008 AppleTV derivative, it's likely got the facility to encode x264 as well as decode, which is an open challenge for hackers, and more of a curiousity for regular users. but i can't see apple touting the ability to rip DVD's or watch Blu-Ray media, etc. when it would be handy for AppleTV HD movies -> iphone re-encodes if it could encode via the GPU, and still keep their DRM teeth attached to the end-product.
and, since it's going to be a flagbearing apple product, it might actually work out of the box for regular non-itunes store / non mobileme / non-iphone users as well. but dont' expect miracles.
i suppose a saturated market didn't stop the appleTV, or the airport extreme, or even the ipod, from the approach of tweaking a regular product and locking it into an apple-only service, but storage is going to be a tough nut to crack, it's still irrationally expensive compared to single-disk solutions like the Time Capsule NAS..
likely what would be intriguing would be a $900 or so diskless mac mini/xserve, with a compatible geforce go chipset, in a decently polished cube case, replete with backlit venting ducts/ports/go-faster-racing stripes in the middle to separate mainboard/HDD heat without being too loud or obnoxious i.e. 160mm blowers or something crazy.
add a few dozen ports to connect firewire/ethernet/USB devices on the back, an apple logo somewhere on the thing, and a door to close over such trivial things as power buttons, drive status indicators, etc. as for design, probably good to avoid the Lacie fetish thing, perhaps the White/Red HAL9000 eye instead of the Blue Hal9000 eye of the 5big model ? sounds good so far. maybe even put in some IR tracking so the backlit grey apple logo light follows you around the room, etc. so, big, expensive, convoluted, creepy, sounds like fun so far.