I think 3.2 was just a stop gap measure for iPad only. iPhone and iPad will get 4.0 when its released.
Bingo.
Anyway, my 2 cents:
Apple is NOT going to majorly overhaul the UI. Look at OS X. 10.0 -> 10.6 has seen tons of refinements to the UI - ditching the stripes, getting rid of the menubar corners & making it transparent, changing how tabs look, etc. - but they haven't totally redone the UI. For one, it'll confuse a lot of people if the interface was to suddenly wildly change one day, and beyond that, Apple chose the original UI for a reason, because it's intuitive.
I do believe, however, we will see 2 major things with 4.0:
- A major overhaul of the home screen, possibly to support multitasking but mainly because 20 pages of apps is a nightmare. I think we may see the introduction of categories or some such and a new method of navigating through apps, perhaps like webOS. The next iPhone's screen is probably going to be larger as well, so expect some new graphics to take advantage of that. Perhaps background images like on the iPad.
- A brand spankin' new App Store, fully matured with sections for all the segments of the market (iPod, iPhone, iPad). Something more like Steam.
Lastly, I'm hoping both the iPhone and iPad get USEFUL locked screens, and an overhaul of the notification system, preferably something like webOS or Android where there is a 'system tray' that tells you what's up, instead of annoying modals.