The Canon Rebel T1i features a 15.1MP sensor compared to the Nikon D90 with a 12.3MP sensor: The common assumption is that more is better and that the Canon Rebel T1i offers an advantage with its higher resolution sensor.
If Canon had opted to recycle the better quality output 10MP sensor from the Canon EOS 40D and incorporate it into the Rebel T1i / ESO 500D we would have been more impressed with their new product direction.
By adding even more pixels without increasing the size of the sensor, Canon is pushing the resolution threshold of APS-C sensor technology and in our opinion has chosen to sacrifice on quality and practicality in favour of a marketing based decision.
In order to fit 15.1 megapixels on a sensor that is slightly smaller in size compared to the DX format sensor found in the 12.2 megapixel Nikon D90, the pixels on the Rebel T1i sensor have to be smaller and packed closer together. The pixel density on the Canon Rebel T1i / EOS 500D is a fairly high 4.5 MP/cm² compared to a pixel density of 3.3MP/cm² on the Nikon D90.
There are a number of disadvantages associated with higher pixel density sensors. One important consideration is that you will want to invest in a higher end quality lens that can support the sensor's increased resolution capability. The simple rule is that if you invest in a higher-resolution camera you need a lens that offers sufficient resolving power, which generally means spending more.
The standard Canon EF-S 18-55mm f/3.5-5.6 IS zoom kit lens for the Canon Rebel T1i does not qualify in this regard, delivering fairly poor image quality when used on a high resolution sensor (noticeable even when used on the 12.2 MP Rebel XSi). We would not recommend this basic zoom lens for use with the Rebel T1i, and would suggest investing in one of Canon's better zoom lenses like the Canon EF-S 17-85mm IS or the Canon EF-S 18-200mm instead.