I have had the pleasure of dining once at Tetsuya's, and it was the most magical culinary experience of my life... four hours and 13 courses of sheer bliss (for a bargain $US65 with the pacific peso and all).
Definitely a highlight if you can get a booking! (it's usually fully booked out for 6-8 weeks, but if you're lucky, someone may have cancelled just before you called...).
The Sheraton on the Park also does a fantastic seafood buffet for around $US30 per head, or for a bit of decadence, check out the Wine Banc near the beautifully restored Westin Hotel.
For a nice summer's evening in Sydney (as it will be for xmas), you can also try out Nick's at the Cockle Bay wharf - nice food, good service, great views overlooking Darling Harbour - a very relaxed out outdoor eating experience.
[This message has been edited by 4me2no (edited 07-19-2001).]