I think you'll find D20 Modern is going to be pretty poorly supported adventure wise. With D&D you can pretty much assume a party of adventurers who will investigate a tomb, save a village, etc, etc.
With D20 Modern, how do you know if the characters are say all members of a SWAT team, just normal town folk, paranormal investigators, etc. etc. There are two many options and an adventure suitable for one group will not be suitable for another.
D20 Modern is great because it is generic and flexible, but those same bonuses are a real penalty when it comes to writing adventures for the public, since you cannot predict what sort of characters the players of any particular group will have.
I suppose thats what Department 7 is suppose to provide the common hook in for Modern adventures, and why WotC made it Open Game Content, but it only partly deals with the problem.
Still its not to say its impossible to write generic adventure suitable to a wide range of characters, just its harder than for D&D, plus you have a smaller number of customers, and adventures always sell less than sourcebooks.... so there isn't much incentive to make adventures for D20 Modern.