OK Fodorites, I realise that this is no American's specialist subject, but I want to treat my girlfriend and take her for a really nice veggie meal while we're over in New York. Can you recommend anywhere?

Sure, there's a great place on the Upper West Side:

Josie's
300 Amsterdam Ave
(at 74th Street)

From Citysearch:
Editorial Review

Teeming with young and shapely bodies, Josie's could easily be mistaken for the commissary of the Equinox health club.

The Food
Wheat grass and carrot juices are served at the bar and vegetarian dishes are the norm in the dining room, where you'll also find grilled fish, tuna burgers and organic chicken sausage. Celebrity partner Rob Morrow and diehard dieter Ricki Lake can be spotted dining here.

I concur with Josie's (it's very good) but you should be aware that it gets VERY crowded and noisy, especially on the weekends.

Sounds like you're looking more for a romantic date place but I'll make my recommendation anyway. Zen Palette in Union Square serves 100% vegetarian meals and has pretty cool atmosphere, just not on the romantic side.

Ditto on Zen Palate though it seemed to be vegan to me, it might be too harcore for non-vegetarians but that's up to you. I love vegetarian meals, and Zen Palate was good, but in retrospect, I would have preferred something with lacto-ovum (or whatever it's called) choices. It's in a great location fright across from Union Sq. Park.