That was not available when the iPhone came out. I lived on the I5 corridor and I can assure you that 3G was NOT available from Sacramento all the way to the Grapevine. . . until about 2009-2010. It was only being introduced in Stockton, the largest city on i5 south of Sacramento and north of the Grapevine in 2009 and I5 formed the western border of Stockton. It was certainly not available in 2006 and certainly not available North of Sacramento. As I stated it was only available inside a few large cities and even then very limited.
You may be right. I just kept my eye on signal strength, not what type.