No, they are absolutely not the same. The Tahitian hips or 'farapu' (please forgive any mispellings!) are supposed to be very fast, but very large movements. I believe the goal is to be large and fast without becoming sloppy - keeping the movement isolated to the hips. So they are similar to a very fast omi, but the better Tahitian dancers' hips cover a lot of area. They should in no way look like a shimmy. This is an example of a beautiful dancer...
www.youtube.com/watch
Please search around youtube and you'll find tons of wonderful examples.
I've been belly dancing for 9 years and Tahitian dancing for 1.5 years and I have to say prior to the Tahitian classes I had never moved my hips to fast in my whole life!

From what I understand, there are also fundamental physical differences in how the dance is performed. Someone correct me if I am wrong, but my understanding of Tahitian dance is that a significant aspect of hip movement is the use of the knees to assist the hips. From my experience in belly dance, this is never the case.

Your question, Rhonda, is a very good one, and to my way of thinking points to part of the problem with fusion dance "adopting" terminology from other dance disciplines. All the dance forms that accentuate the hips are not the same; they are not necessarily interchangeable (if you wish to remain true to a technique); and they certainly do not warrant being terminologically malappropriated.

OK, that's my rant for the day!
;>

I do both of these dance forms, and no, a bellydance shimmy is not the same thing.