According to the Wikipedia page:
http://en.wikipedia.org/wiki/Ford_Ranger
Sales decline, Rumored discontinuation, and the Rebirth
Ford chose to invest improvements in the Ford Explorer SUV which was branched to a more advanced platform than the Ranger, letting Ranger's sales decline. A similar strategy was applied to the Ford Taurus, another former best seller which was replaced by two new nameplates rather than redesigned. However as a result of the drastic shift toward smaller and more fuel efficient vehicles in North America, Ford has said it will continue to produce the Ford Ranger through 2011 at its Twin Cities, Minn. plant, which was scheduled to close in 2009.
Since then, all of its competitors from the Dodge Dakota to the Toyota Tacoma have been redesigned and enlarged towards the mid-size market (with the Dakota offering a V8 engine) and leaving the Ranger and its twin, the Mazda B-Series, the only compact trucks on the market. For example, the 2005 Nissan Frontier offered up to 28% more power than a Ranger (265 vs. 207), and the entry level model was over a foot longer.[8] Since 2004, Toyota Tacoma has taken a commanding lead (93% higher sales than the Ranger in 2006) in the segment. In January 2007, Ford sold 4,652 Rangers, a decline of 25% from the previous year, one-tenth the number of F-Series trucks, and about half as many as the Explorer.
Contrary to rumors of its demise, MotorTrend confirms that "Ford is working on two new, smaller-than-F-150 pickup trucks, one based on the F-Series and the other a replacement for the aged Ranger compact, which is unchanged, save for facelifts, since the 1998 model year.
The smaller F-150 is codenamed P525 and may be badged F-100, the name of the F-150's predecessor from the 1950s. At one point, P525 and the Ranger replacement were competing proposals, but with growing need for higher-fuel-mileage trucks, both projects will become reality in 2010 or 2011.[9] The P525 is rumored to have the V-6 EcoBoost (gas direct injection and turbocharging) engine as its primary engine, with fuel economy and torque optimized for interim Corporate Average Fuel Economy standards expected to kick in for the 2011 model year. The second engine which may not be available at release is a new small modular 5.0 liter V-8 featuring Ford's version of displacement on demand.
The Ranger replacement, codenamed T6, is believed to be in development by Ford Australia and was originally intended only for world markets other than the U.S. Some news sources of indicated that Dearborn has reconsidered that plan, and that Ford thinks high gas prices are definitely here to stay.
The plan for F-100 was put on hold, instead the company decided to offer EcoBoost engines for F-150. The new truck was to have been built at the Michigan Truck Plant in Wayne, but Ford has decided to retool that plant to produce small cars. It could still build the F-100 at one of its other truck plants if it later determines there is a need for the product.