SO-DIMMs are notebook memory, but if you consider the size/shape of the Imac, its about the same size of a laptop (maybe a little thicker), so SO-DIMMs work perfectly for it. They also use less power, which goes hand-in-hand with the low energy consumption of the core duo processors.
The description of the Intel Imacs on Apple's site say it uses SO-DIMMs, so I wouldn't worry.