Dual Boot screen is not coming after installing win 7 via boot camp!
The Mac will not show the Startup Manager screen to you every time you boot. You can set a "default" partition to boot from, or hold "option" while it boots to get the Startup Manager.

To change the default, you use "Startup Disk" under System Preferences for Mac, if you are on Windows you can use the Bootcamp Utility in the system tray.

AFAIK, I don't think there is a way to get it to show the Startup Manager every time you boot without holding the option key.