Replace secondary SSD drive...

If you buy the latest iMac with the additional 2.5" SSD (3.5" HSS +2.5" SSD) you can replace by your own the 3.5" HDD with a 2.5 or 3.5" SSD (and the 2.5" default SSD with an other 2.5 HD as well)...

otherwise, if you buy it without the additional SSD you will still have

the extra space for a 2.5" HD (SSD or HDD as you prefer) but you will have to buy the proper support to build it in...

Does anyone know if the extra SSD space of mid 2010 27" iMAc is also in late 2009 iMacs? In other words:

I have a late 2009 iMAc and I would like to add an SSD without replacing the combo DVD... do you think it is possible?

At the moment I think I'll substitute the HD with an SSD...

It is indeed possible to add a secondary SSD to the 2010 27-inch iMac, utilizing the third inboard SATA connector which those models have. I'd suggest the following link:

http://ssd.youds.com/

The author provides suggestions on parts and presents fairly solid teardown and upgrade instructions for the 2010 27-inch iMacs. I used the above site as a reference during my own 27-inch iMac SSD upgrade and found that it is quite possible to avoid purchasing the SSD bracket for installation in the same fashion that Apple employs on build-to-order models with an additional SSD.

Unfortunately, Pietro, the 2009 27-inch models do not have the third SATA port and thus can only be upgraded by the replacement of the optical drive or main 3.5-inch hard drive, as you are considering.

Hope that helps!

Jason.