No. Thats not a hard and fast rule. A simple google search will tell you that it can be upgraded, sure compatibility and socket type issues are there, but as he said, his MoBo supports the newer processor.
Just one of the google results, How to Upgrade an Intel Processor Laptop | Tech Tips - Salon.com
Speaking from experience. Not mine, but a friend's.
And from my experience, when I bought my Dell 5.5 years back, it came with a faulty MoBo. The repair guy in my campus changed the MoBo in front of me. He just changed the MoBo and put all the other components (including my C2D processor) from t he older MoBo. Thats how I know it can be done. Though this was a case of same processor going on same MoBo, if sockets and pins are compatible, other processor can go in too. Even if its "integrated" and not "screwed" it can be changed, though not by yourself but at a repair shop who do chip level servicing. This I know cause I have had my 8600M GT gfx card chip replaced twice in the past 1 year, they de-solder it out (something called BGA level repairing) and fix the new one.