I agree, it is relatively easy to install such an adaptor in case of 3.5 inch HDDs - there is alwas space available in desktop computers. However, in case of laptops this will not work due to space limitations. The only possibility I found so far involves soldering (written in German):
http://thinkpad-forum.de/threads/99705-Mod-SATA-Festplatte-intern-mit-bis-zu-1TB-für-T40-p-T42-p?s=1d8ab2995786ec3c5e50f3f01ae812bf
