The New Canon 550D is an 18 megapixel beast, but like Shania Twain said, that does not impress me much. It has a sensor size of 22.3 x 14.9 mm, and at full resolution, each pixel on the sensor resolves to around 4.3 µm. Compare that to Pentax's K-x which resolves to 5.5 µm, more than a full micrometer larger. It may not sounds like much until you do the math and find that each pixel on Pentax's K-x is about 29% larger than the new Canon 550D.
Strongly recommended read: Petavoxel's Megapixel Madness.