What is the point with iMac + SSD? The newly released iMac can be built with an optional 256 GB SSD. To complement this, the new iMac's also contains the Intel Z68 chipset:

Z68 mproves performance by putting files
that are accessed often on the SSD.
Intel is coming out with a small SSD (20 GB), large
enough to hold the Windows 7 operating
system. This improves performance up
to 355% depending on the application.

It strikes me as odd that Apple uses a (very expensive) 256 GB drive when you only need about 20 GB to get the benefit.

My late-2011 MacBook Air (which has SSD) is incredible snappy and I wonder if an SSD-iMac would see the same improvement?
Why use the 256 GB drive when a much smaller and cheaper would do it?
The base model of 21" iMac only has one drive bay. So, a 20GB SSD wouldn't be very useful there. If you want an SSD, you have to get one big enough to hold all your data.

As for the 27" iMac and the upper model of the 21" (which have two drive bays):
Performance. Yes, you can fit the OS on a small SSD, but that's not the only thing that will benefit from being on a faster drive. Any operations that involve a lot of reading/writing will be faster if they're working with files on an SSD.
Convenience. Unless you set up your apps and Finder especially well, it will take a little more time to get to stuff on a secondary disk.
Quietness: SSDs don't chatter like HDDs. If you have both an HDD and SSD, you'll still hear the noise. Silent computers are nice, IMO.
Apple-ness: One of the things many people like about Apple computers is that things are mostly seamless ('Just Work'). Having one big, fast, quiet drive is more seamless and Apple-y than one small, fast, quiet drive and one big, slow, loud one.

One more reason Apple does it: Because they can: Add SSD. People buy SSD. Profit!