Disk Speed Test notes What's New Version 2.1: Some SSD's use hidden compression when writing data to make their benchmarked speeds appear faster. Disk Speed Test will now measure the true speed of these SSD's so you know if they are suitable for high quality uncompressed video capture.
So what does this mean? It means that Disk Speed Test from now on tests with non-compressible data. Before version 2.1 it tested with very well compressible data. The Vertex 3 does on-the-fly compression and reaches 500+ MB/s sequential writes that way. However, if the data is not compressible at all, the Vertex 3 manages about 140-150 MB/s sequential writes. This is normal behavior.