As standard you can drop down one contract 'level' a month after 9 months (so you can't go from £75 to £30 in one jump, but £45 to £35 is OK).
The iphone specific stuff I guess we'll only hear a few days (or on the day of) launch... can't see them changing that part though. Their current iphone contract doesn't mention the 9 months.. but the 2G iphone wasn't subsidised so they didn't need it... with the new one they'll definately have this limit (otherwise you could get a free iphone, then drop once a month to £30 and they'd never get their money back). It's in the first paragraph of the standard contract though.