An interesting (and confusing) topic guys... and good to see a discussion on crop factors that doesn't just turn into a debate on sensor sizes
Reading Toad's and Slej's posts, I get the feeling that both state the facts correctly, but I'm still left thinking the perspective aspects are a bit unclear regarding ST's original question about finding an "equivalent" standard lens to the 50mm on a full-frame.
AFAIK, no matter what lens you use, if the camera and the subject and the background all remain in the same place, the perspective will *always* be the same. It doesn't matter what focal length or sensor/film size or "crop factor" you are using. Perspective is purely a property of the physical spacial relationships between the camera and elements of the photo*.
So in order to get the same perspective as a 50mm lens on a full-frame camera, you simply need to be standing in the same position when shooting. Of course if you are looking for the equivalent to the 50mm lens for APS-C cameras then you'll also want something that gives the same field of view as a 50mm lens, and in this case a 35mm lens would be the closest for people with a crop-factor around the 1.5 mark. So... yes! a 35mm lens will give you the same perspective as a 50mm lens on a full-frame!
But as the others point out, that's not the whole story. The depth-of-field will be different between the two lenses, the bokeh will be different, and there are a number of other things worth thinking about.
All lenses usually provide optically inferior results towards the edges and corners of a a photograph. Vignetting, a loss of sharpness and chromatic abberation are all common undesirable properties that can creep into photos towards the corners. By cropping the image circle down, photos from an APS-C camera use more of the "sweet spot" of the lens and less of the corners, which can greatly reduce some of these optical problems.
The flipside of this however is that the resolving power of the lens needs to be greater in order to maintain the sharpest possible image, as the APS-C camera is effectively "cropping and enlarging" the centre portion of the image circle when producing a photo relative to a full-frame camera**.
But finally... my real question is... Does a 50mm lens really represent what the human eye sees? And I'd suggest the answer is NO. Well not any more than a heap of other focal lengths anyway.
In fact, when I think about what I can see, I notice that my field of vision covers well over 90deg horizontally. Of course I can't make out sharp detail at the edges, but when looking at a spectacular landscape the entire view adds to the impact of the scene.
Even my 10mm lens doesn't go that wide (on an APS-C), and it is 5x wider than a 50mm lens or 3.5 times wider than a 35mm.
And have you ever noticed how when you take photos of the moon, it always comes out a lot smaller than the way you saw it with your eyes? Try taking a moon photo with a 50mm lens, it'll be a little white dot in a big sea of black. I think you need around a 150-200mm lens (on an APS-C camera) to re-create the feeling you get of looking up and experiencing a big moon in the sky.
But.. what's going on? Our eyes don't have zoom lenses, how come I'm suggesting we'd need a 10-200mm zoom (on an APS-C) to approximate human vision??? What's going on???
Well... I simply believe it means we see with our brains, not our eyes.

Our brain can "zoom" its attention in and out in a way a camera can't. In photography we use different focal lengths to lead the viewer toward whatever it is that we had our attention on at the time. And I don't believe there is any standard level of attention, so therefore no standard focal length.
Of course... this is all just my unscientific opinion.
Cheers
Adrian
* I think this is complicated slightly by some ultra-wide angle rectilinear lenses and fisheyes which introduce some distortions to try to best represent a large 3D scene in a small 2D photo, but these are not the same as real changes in perspective (although they are often referred to as things like "exaggerated perspective").
** Its not really enlarging the image when it captures it, but when you print it out at a standard size (ie 8x10") then it will be effectively enlarged more than a full-frame image printed at the same size... because the enlargement factor is always relative to the sensor/film size.