Apple updated Aperture to 2.1.2 today, but alas, no support yet for RAW files from the Nikon D90. RAW files are a capture of the data right off the camera's sensor, and are so-named because the files are not yet processed into something that can be viewed or printed. RAW files are usually something only pro photographers or at least "prosumers" are concerned with and are only output by DSLR-type cameras, but recently even high-end compacts like the Nikon CoolPix P6000 or the Canon G10 support RAW.
Camera makers include proprietary software with their cameras that can read their own RAW file format and display or edit it, but the included software is often less-than-stellar in quality. And despite the drawbacks like the proprietary nature of each RAW format (Nikon's NEF, for example), and the large file size (usually twice the size of a high-quality JPEG file) you want to use RAW when you want the highest level of control in post-processing the photograph, since so much detail is available in the RAW file in the first place. So, people turn to software programs like Apple Aperture, Bibble Pro and Adobe Lightroom. It must be a huge challenge for Software Makers to keep up with RAW formats. Here's hoping that Apple puts Nikon D90 Support in version 2.1.3.Update - after further research and to be accurate, Apple updates its RAW support with "Digital Camera Raw Compatibility Update" software updates such as this one, which either come as standalone updates or as part of an OS X update. See "Digital Camera RAW Formats Supported" and Aperture's RAW Support Page. So, this is not an Aperture issue per se, as Apple has built RAW support into the Digital RAW Compatibility module. Maybe the fact that Adobe Lightroom has been updated to support the D90 will help motivate Apple.