Here is a 30 minute research into the jobs out there for RoR compared to Asp.Net & PHP. I searched for the phrases "Ruby on Rails", "PHP", "ASP.NET" and radius of 100 miles from the zip code. There is no doubt some overlap of job posting but this is pretty clear indication of where things are.

The three locations he picked were Cincinnati Ohio (his hometown), New York City and San Francisco and the three sites were Monster.com, Career Builder and Hot Jobs. Overall, the postings varied from place to place - ASP.NET cam out on top in Ohio, ASP.NET in New York and PHP over in San Francisco.