Silicon Valley encompasses many cities in a "region". Here in Detroit, we don't have a name (yet?) to describe the region so we use Detroit to encompass the whole area. Ann Arbor falls within that area. It's roughly a 45 minute drive from Ann Arbor to Detroit...or 35 minutes for me ;)
The Detroit "region" or sometimes known as "Detroit Urban Area" or "Metro Detroit", is actually quite large and has many beautiful communities with in it. This wikipedia page sums it up well. The "region" is actually the 9th most populated urban area in the US.
http://en.wikipedia.org/wiki/Metro_Detroit
People also forget how large the actual city of Detroit is. If you look at the map linked below, you'll see that Boston, San Fran, and Manhattan all can fit within the city limits of Detroit.
http://markmaynard.com/?p=56