The A-10 was originally designed as a tank buster to stop the Soviet army in an all-out assault through central and western Europe. It was literally built around the powerful 30mm cannon, since with a tank killing cannon, one aircraft can kill a dozen or more tanks, in addition to however many it can get with external ordnance such as cluster bombs and AP rockets.
As was mentioned by many previous posters, the USAF was planning to retire the A-10 and replace it completely with the F-16 when Desert Storm happened. The Army was pitching a fit over this and the USAF apparently offered to hand the A-10 over to the Army. Desert Storm changed all this. The A-10 was one of the leading killers of ground vehicles during the war and actually scored 2 air to air kills (contrast this with no air to air kills by the F-16 and only 1 by the F-14) against Iraqi helicopters. This makes it a tie with the F-18 as the 2d leading air to air scoring type of the war. Its greatest contribution was in the air to ground role in the CAS mission however, just as it was designed to do. The A-10 supported USA and USMC troops and also hunted Scud missiles. It also gained the role of forward air controller as the OA-10.
The USAF revised its date for the service life for the A-10 to 2030 which was as far as was projected at the time. Intervening years, changing technology, and 2 subsequent wars have changed things again however. The F-16 and the A-10 are both slated to be replaced by the JSF eventually. Smarter enemy weapons have made even the super tough A-10 almost unsurvivable in any really hot combat environment. Several A-10s were lost in Desert Storm from ground fire and SAM and AAA technology has advanced significantly since then. Shoulder fired missiles have improved from merely a nuisance to very serious threats to even as tough and manouverable a plane as the A-10. Within 15 years, any opponant that we are likely to face that is much above the level of Afghanistan, will almost certainly have the most modern Russian missiles. There are SAMs for sale right now by Russia, that are of superior capability than our Patriot missile was during Desert Storm. Missiles like these make at least some level of stealth an absolute necessity. The A-10 has none.
Another factor in the replacement of the A-10 is the development of the B-1 bomber into a tank killer par excellence. The new smart cluster bomb which actually has skeets that linger over the battlefield before each choosing a target and hitting it precisely, can be carried in great numbers by the B-1. A pair of B-1s flying low over a battlefield at supersonic speed, can abolutely decimate an enemy armoured force. The effects of this type of attack were negligible before if the enemy dispersed their columns even a little and didnt stay bunched up in open areas. The new smart munitions have changed all that however.
The A-10s are wearing out. They fly in a high-g environment and the parts for them are just not made anymore. The B-52 is older yes, but the B-52 fleet spent much of its life sitting around on nuclear alert. It is only in the past dozen years or so that the B-52s have been tasked as heavily as many other aircraft types.
The A-10 will probably soldier on in active service until enough JSFs are produced to replace it, probably not until 2015 at the very earliest. Some may remain around in the OA-10 role, and many will certainly remain in the ANG and reserves. Some of the retired A-10s may find new life as firefighting aerial tankers as several proposals have been floated to use surplus A-10s as firefighting aircraft in the west to replace the old piston engined large aircraft that they are using now. The A-10 with its manouverability and lift capability will probably excel in this role and may be around for many more years yet in this role.
The Air Force is trying to consolidate into as few aircraft types as possible, to cut down on costs of maintenence and parts, and to simplify logistics as much as possible. The USAF of 2030 will probably consist of the following:
-F-22 for air superiority as well as some strike missions, most likely in the role of reducing enemy air defense networks
-F-15Cs in ANG service for national defense
-F-15E for deep strike after air defense systems have been degraded
-F-35 JSF for ground attack, CAS, and other strike roles
-KC-10 tankers for refuelling and heavy lifting
-C-5B the best of the current C-5s for heavy lifting
-C-17 for all round lift workhorse
-C-130J for intra theatre transport and tactical transport
-B-1 strike and interdiction
-B-52 nuclear deterrance and general bomb truck
-B-2 primary bomber in initial attacks, nuclear mission, all high threat strikes
-undetermined new tanker, possibly based on the Boeing 767.
-some undetermined as yet, electronic aircraft that combines current AWACS, JSTARS, Rivet Joint, and ABCCC, possibly all on the new airborne tanker platform.
-Airborne laser for shooting down ballistic missiles and conceivably enemy aircraft as well.
-an unmanned combat vehicle for high threat environments.
-Global Hawk and other UAVs to handle all recon roles
-an as yet undetermined wild weasel/EW aircraft, possibly a new UAV or an agreement with the USN to have joint squadrons of the EA-18 similar to the current arrangement used with the EA-6b.
Airplanes currently in USAF service that will most likely be scheduled to go away by 2030, at least from regular service:
C-141, some older C-5s, some older C-130s, F-16, A-10, F-15 A/B/C/D, F-117, KC-135, possibly AWACS and JSTARs in their current form.
