Apart from Auto WB bracketing feature (which replicates total of 3 shots when shutter button released once), photographer literally need to take 3 (or more - see below) continuously shots of the same subject for HDR post processing with the Nikon D90. Note to shoot RAW please.
Hence, a tripod is highly recommended. If you want to cut corners or when tripod is not available, you are advised to shoot at the maximum frame rate offered by the Nikon D90 - 4.5 FPS. Once image is composed, press the shutter to salvo the scene at max rate to reduce shakes.
A new discovery - the overlay tool in Nikon D90 cannot clearly optimize the overlaid images, hence you need Photoshop, Photomatix or something similar.
Some believe a total of 16 shots are to be taken on a particular scene while some chanced it at 2 to achieve optimal HDR images. I am doing 3. (As I am not very motivated to do 16)