I think the smoothness of iOS has quite a bit to do with extremely tight hardware integration, a very limited feature set, and a very, very strict eye for UI aesthetics.
I think there are 2 major issues that really affect the different; Age & Many Manufactures.
Apple has been developing computers since the 70s. The first Iphone was introduced in January of 2007, in July 2008 came the 3g and June 2009 came the 3gs followed this June with the Iphone4. Apple controls the hardware, software, everything about the devices and in the last 4 years, they really have only released 4 devices that aren't that much different.
Android 1.1 was originally released on February 9, 2009, merely 18 months ago. It is new but developing fast. Unlike Apple that tends to do a single software/hardware release each year, Android had 5 released in 16 months and Gingerbread scheduled for 4Q 2010. In that same time period Apple had only released 1 Iphone.
It is not a closed platform, there are hundreds of devices being released each year from almost as many manufactures, some better than others but none from the same company developing the OS. Some of these devices are similar, but some aren't and they have to deal with those differences.
I personally think that it would make things more stable if the OS releases weren't so close. (1.1 1/9/09 1.5 4/30/09 1.6 9/15/09 2.1 1/10/10 2.2 5/20/10), but they do seem to be slowing down a little bit
It would also make it easier for the Manufactures to develop devices and not have to worry about releasing a device on an OS that is "outdated" so soon.
