Same thing going on here. My desktop and media centre used to be always on. Now I've sorted out standby mode properly (3W in standby, wakes in about 4 seconds), and just the server stays on permanently.
My server was at its peak using 11 drives: 5 x 400GB, 6 x 500GB, which made quite a bit of noise, and used quite a bit of power. When I worked it out, I reckoned there was about 50W power to be saved. I decided to get rid of the RAID configuration, and replaced them with a pair of 1.5TB greenpower discs. A small cut in capacity (3.6TB down to 3TB), but it cut the machine power usage by nearly 60W - over £60/year.
I've got a box of 14 discs that are now just offline backups, although the server now also has a pair of 2TB greenpowers also.