I've been using a 500GB WD Elements drive since last April & have never had it spin down even when letting it sit for an hour or more. I've had this drive around 18 months though so it's hard to say if newer models would have that problem or not.
Got the drive and it does not seem to spin down, however it does "park" its read head after a few seconds of inactivity, and when game requires data the hdd will lag a bit when it moves the read head out of park area. the hdd smart readings' load cycle count increases very fast due to this.
it's not as bad as spin down the entire drive but the loading hiccup gets really annoying, looks like i'll just keep this drive as storage and get a Seagate freeagent for Wii games.
Got a WD Elements 640gb, had it for months, played about 50 games on it, played for over 12 hours straight frequently, never spun down for me on any cios rev. It's an uber drive and I'd heartily recommend it.