The first d20 Modern style prestige classes I am aware of are the ones found for the Genetech Setting in the January issue of Dungeon/Polyhedron. There you will find the Guerilla Intruder and the Patrol Pointer. The two PrCs follow the 5 Level format.
Morrus has Urban Arcana already, you could ask him for confirmation of the PrCs in that book.
I have my copy. I paid an arm and a leg but budgeted for it so, meh. Going through it now. I like it, but with reservations. I know the book is split rigidly between flavour and crunch but for the price I would have really preferred to not have to do any of the work to make the setting npc's. Descriptions like Lamberto: (Tough3/Dedicated4/Soldier3 Hidden Sun) and a bio cheesed me off just a tad. No stats at all, no mention of birthright choices.
Will start the game next month we'll see how it goes. My initial impression though is that while I was hoping to base my campaign on this book solely they didn't provide me with enough to do so, not for the price anyway. I'll wait to see if they make up some of the shortfall on the web.
Personally, as a GM I'd call it an average to below average effort as an RPG resource. Aside from the cover, the artwork inside didn't do much to sell the setting.
This isn't a proper review by any means and I didn't really even mean to start a half-assed one but as I was answering the question about PrCs...well stuff just came up (shrug).
Keep in mind I paid nearly CAN 60$ so I was genuinely expecting some more bang for my buck. I would definitely be interested in others opinions on the book.
Mythic Justice, any plans for Web Enhancements?