IMHO the 550D has some really well designed 'quiet pc' features.
No significant holes in the front, top or side - check
Twin material construction, steal and aluminum vibrate at different frequencies - check (quality of juctions become very important)
Minimal holes to rear - check
Door to hide / reduce noise from optical drive - check
Indirect sound path for front intake - check
Option for passive stack (fanless) opperation (vents on base / roof) - check
External passive watercooling gromits - check
Sound-proofing material (?) - of limited value but check
Magnetic dust filters - check
The psu is in the wrong place - if it was at the top you could remove all the fans except the psu (extract) - but you can't have every thing.
room for a low fpi rad in the roof (run passively) - maybe...
Would make a really good base for a 2 fan system (psu and rear extract)
or a one fan 120.2 semi-passive watercooled system.
It's not a SLI / Crossfire case - but an inaudible SB / 560gtx / 80+ gold psu = easy
In direct compertition with the Antec P280
Send me a sample and I'll prove it