That is what the marketing talk says. The greatest speed increase will be for video performance, which is what the Cell is optimized for.
At this point, what seems to be hampering the Cell is unavailability of the Nvidia graphic processor. Early demos have been conducted without it (and have still been impressive). Sony funded Nvidia to develop a new graphic processor specifically for the Cell. Personally, I think IBM and Sony should have contacted two or three video card companies and used all of them as sources, to avoid putting all of their eggs in one basket. It may make sense for IBM to aquire a video company; Matrox stands out as a good candidate (Matrox's 3D stuff would need some work, but that's feasible).
We'll just have to wait and see what happens. I'm looking forward to it. If the Cell turns out to be as fast as expected, word is that other microprocessor companies will immediately copy the architecture.