This is a known issue concerning the D90, and at least the D70 as well, that has been discussed at length on several forums. The problem usually seems to be with the on/off switch.

Many users have reported that moving the power switch all the way over to the lightbulb position for lighting the top-of-camera LED display and pulling their finger back and letting the spring loaded switch flick back to off solved the problem for them. This seemed to work more often when the user allowed for the possibility dirt or dust had been able to make its way into the switch.

Others have used a more methodical approach:

1. Turn camera off
2. Remove SD card (SD card remains out until step #11)
3. Remove battery
4. Turn camera on (without battery)
5. Turn camera off (without battery)
6. Replace battery in camera
7. Turn camera on
8. Try two-button reset (no visual indication this worked)
9. Use menus to reset custom settings (see #8)
10. Turn camera off
11. Replace SD card
12. Turn camera on

Some have reported the lens must also be removed for this to work. It seems that usually it is a power switch issue, but if the contacts for the SD card or lens are loose the camera will blink on and off each time it thinks either the SD card or the battery is removed or replaced.

Others have suggested that inserting a freshly charged battery immediately after removing it from the charger sends too much voltage to the Main Power IC Board in the camera and damages it. Waiting a few minutes after removing a fully charged battery from the charger is suggested as a preventative measure.

If you google Nikon Green Light of Death, you can see many more links to forums where this has been discussed.

If neither of these methods solve your problem, it is probably time to consider sending it to an authorized Nikon Service Center for a repair assessment. If the cost of repairing it is a significant percentage of the value of the camera, it may be time to consider replacing it instead. Good luck!