Was just talking to a friend who owns a really early (2006?) MacBook Pro. One of his friends has just bought the new MacBook Air that I'm currently drooling over and he was really impressed with how fast the notebook is despite the fact that it should be slower on paper than his old MacBook Pro.
So in an effort to breath a bit of life into his old laptop he fitted a 40GB solid state disk for the OS and apps and fitted a 640GB hard drive in place of the DVD-ROM (which had broken anyway he said) for his data. The result? A five year old laptop that boots Snow Leopard in around 15 seconds from power on and loads apps like Firefox in under one second.
Now that prices are dropping and sizes are increasing fitting an SSD is a good way of perking up an aging laptop - I'll certainly be looking into fitting one to my nearly 4 year old MacBook.