The question is why does Samsung make this decision? Simply because of the software issue. In those devices Samsung has put lot of big-sizes of software to which those devices will lack enough ROM and RAM if having the ICS installed and run.
Of course this decision will raise a wide disappointment among the Galaxy S users who have deep expectation that their 1GHZ Humingbird-based phone could handle the ICS, let alone its sibling the Nexus S getting the ICS via OTA albeit they both are using the same hardware.
And Samsung explains to that possible query and dissatisfaction that the Nexus S is qualified to receive the ICS it is because only running the basic Android OS, while the Galaxy S to deliver carrier services, TouchWiz, Mobile Tv, video calling software and, TouchWiz.
But looking at the other side, surely it's about Android evolution and it can't be retained simply because of the "old" devices holding it to evolve further. This is indeed unhappy news. However now you have three options on the table, upgrade your hardware, or stick with what you are holding, or using the unofficial porting -- install Android ICE Cream Sandwich to Galaxy Tab.
Good day guys.