The variable factor appears to be the southbridge.

Most laptops don't actually have a SATA-2 controller. They are compatible with SATA-2 drives but they don't offer SATA-2 speeds.
A desktop will always offer better performance. The 240 MB/s figure is what some people were reporting in the OCZ fora with earlier (less stable) firmware versions.

On a positive note though, the Vertex drives now have a longer warranty.