I think d20Modern high levels campaigns are more along the lines of your power level in the world as opposed to just your character level. In other words, situational involement, where your characters can get into at 3rd level is allot different than 13th level, and so on.
My last D20 Modern campaign, and soon to be intertwined with my new one, was a solo campaign that started at 3rd level and has moved up to 17th level. Now Jack started as a Bond type character in England at 3rd level, he had access to 2 contacts, a car, a place to live and limited resources. At 17th level, he has dozens of contacts, is freinds with Kings and Presidents, has a castle and a half dozen other residences or places he is always welcome, and his resources are a little crazy
. Power levels way up there at this point.
So as he goes up so does the competition and power level of the entire playing field. I wasn't afraid to let him clime the power ladder, I included many known personalities and famous types to give it that higher level feel. Meetings with the heads of governments, powerful celebrities and politicians are all fair game in my campaigns.
Alaska