No, but we do need a setting for the game rules. After all, that's how D&D was promoted and rose to the success it now has been ... well, up to 3rd Edition.
Later on we can add new settings for people who want something different.
Personally, I'd start with Dark*Matter.
Dark*Matter is a good choice. I found the d20 Modern book quite interesting and it went along the lines of what I had envisioned for a d20 Modern campaign. I prefer it over Urban Arcana.
You're also probably right that we need a setting in the first place. A game without a defined setting will always have it more difficult. You want the game to be supported, and support in a vacuum is harder. You can give rule options for various campaigns, times and settings, but this will only attract those that still need one. If you give players an interesting setting, they will want books on campaign details, new adventures and so on... And the existence of those will attract new players, too.