said by DarkLogix :

Ya but would/could a single server have that much capasity?

in windows 64bit the largest logical drive you can have is 7.5TB so 7.5*26(letteres in the alphabet) 195TB
not lets assume not windows
500TB=minimum 334 1.5TB harddrives (but of course add redundancy so) 336 drives if one large array is even possible

It would be easy to see its possible but it would take some planning as to how to orgenize and various other aspects and possibly a need for ZFS

you could actually fit the drives all in one 48U Rack with space to spare

it would be cool to get more details as to how they set it up(although I know how I would)

said by KodiacZiller :

They won't be using Windows (but knowing cable companies, I wouldn't doubt they'd try).

Like you said, this is a job for a UNIX variant and ZFS (most likely Solaris). ZFS is as scalable as you want it to be.