You're right, given similar megapixel numbers. The Nikon D700/D3 have probably a good two extra stops of usable ISOs over the D300 because they are larger sensors but still only 12mp.
Samsung came up with a way to minimize the spaces between the photosites in the sensors and was able to make a 14.6mp sensor (as used in the Pentax K20D) that has the same size photosites as a 12mp sensor, hence its similar ISO performance to the D300 sensor. I can only assume that Canon used similar tricks with their 15mp sensor. It'll be interesting to see what people say about their cheaper lenses - at that kind of mp level, you can really notice the difference between a cheap lens and a really good one.
Now, if you're looking at a FF sensor that has way more pixels, then you probably won't get that much extra noise performance.
IMHO, the FF lust that infects some people is mostly psychological. APS sensors are capturing levels of detail and low noise levels that are pretty amazing nowadays - FF means larger and heavier (to say nothing of more expensive) cameras, some of which can do higher levels of ISOs - but at what point do you really need more ISO performance? Quality 3200 ISO performance is pretty much a given on the latest DSLRs. One big advantage is that it's easier to do very wide shooting since you don't have the crop factor to worry about. On the other hand, telephoto shooting means that you'll need bigger and heavier lenses to match what you can do on a crop-sensor camera.
