"Full Frame" vs crop factor
When affordable DSLR's came on the market a few years ago, one of the big complaints by many nature photographers was loss of their formerly "wide angle" lenses. Because of the nature of the optics and mechanics, 35mm based lenses were not "designed" specifically for the sensor sizes.
A number of lens mfgrs have started to market "digitally" optimized lenses which have been designed for the crop factor that is on many DSLR cameras. Of course, not every sensor size is the same, which means the crop factors vary, though perhaps not significantly.
One of the new "downsides" of those lenses is how they will act on a "so-called" full frame cameras. For example, the newest Pro-offering from Nikon (the D3) is a so-called full frame camera and shoots at something like 12 megapixels. However, if I am reading their specs correctly, it switches to the crop factor when the lenses designed for "digital" is mounted and reduces to less than 6 megapixels. While I know there are megapixels and then there are megapixels, I am not sure I personally want to give up extra to achieve so-called "full frame."
I say "so-called" because whether its full or not depends on your perspective. On a 35mm film system with lenses designed for that system -- a 35mm rectangle is "full frame." On a Medium Format camera, its tiny. My D2 takes the full frame rendered by the sensor. Its just that I use lenses which were originally designed for a 35mm rectangle. I have changed my equipment to meet my shooting needs (personally, I have a 14mm, a 60mm micro, and a 300mm f2.8 fixed and one zoom--a 12-200VR). That is approximately the same wide range I had with film and slightly longer on the long end.
In my mind, unless I wanted to go much wider, I couldn't see the need for "full frame" for my personal shooting needs (though it might be interesting to mount the 14mm on one and see).
At the end of the day, I think its great that we have all these offerings -- and each person will need to equip based on her/his own shooting style/needs