so the Nikon 17-55mm lens is true 17-55 but Tamron isn't? hmm
No, both the Nikon and Tamron lenses need to have a 1.5x multiplier applied to the specified focal length to get the 35mm film-equivalent focal length on a DX body. The Nikon 17-55/2.8 will get you the equivalent of around 25-83mm on a DX body like the D90. It's listed as a DX lens, so it means the lens' image circle is made for the smaller DX sensor, which equates to a cost saving compared with FX lenses that cater for a larger image circle.
I would respectfully disagree with the earlier poster on whether a Nikkor lens is worth the money over a third party lens. I've never tried Sigma lenses, but the Tamrons I've used have been glitchy (causes errors all over my LCD screen, locks up in AF, etc) and have struggled to resolve anywhere near as well as Nikkor lenses. The question, then, is what price would you put on reliability and quality?
I would give prime lenses some consideration. They'll really sharpen your skills and they're considerably cheaper, too. The Nikkor 35/1.8DXis very affordable; or you might want to try something a little wider like the 24/2.8 (equivalent to 36mm), which approximates a classic wide perspective.
