90 of top 200 sites have no mobile version. Opportunity?

I don't know that this actually makes any sense.

Are we still doing os checks and serving different pages based on the client (I'm not).

With modern browsers on smartphones, I think this is mostly not-necessary for the average top 200 site which likely still targest the lowest common-denominator browsers. Mobile browsers are over-powered and very capable to display these sites.

It's the smaller early adopter sites that are pushing the envelope and may need to do some sniffing.