DOD has released its FY2007 DoD Statistical Report on the Military Retirement System. It can be found directly at http://www.defenselink.mil/actuary/statbook07.pdf or from their Actuary website http://www.defenselink.mil/actuary/.
Previous reports back to FY2001 are also available along with other reports from that office.
If you are not familiar with this report, it is pretty comprehensive providing information on all kinds/levels of military retirement information. Most is available by service and rank. While most of the stats are for the current fiscal year, there is one table for the number of military retirees receiving retired pay by year going back to 1900.
Also included are stats such as:
Military Retirees outside the US
by Zip Code
by rank and current age/age at retirement/years of service by gender