The good thing about sensors smaller than 35mm format is the multiplier effect for the focal length.
For example, with 1.5x (Nikon) or 1.6x (Canon-consumer) or 1.3x (Canon-pro/photojournalism) sensors a 100mm lens becomes 150mm (Nikon) or 160mm (Canon-consumer) or 130mm (Canon-pro/pj) respectively.
The converse is also true (the bad side of the multiplier effect) for wide-angle lenses makes your wide-angle lenses not so wide-angle. So 28mm becomes 42mm, 45mm and 36mm respectively.
Hence, the really expensive super wide-angle lenses now.
Anyhow, I always recommend buying a lens that will work with full format sensors, so no need to change in the future when full frame sensors are the norm.
I have only one lens which won't work with full frame cameras, that's the Canon 18-55 kit lens which came with my Digital Rebel. I bought the kit for less than the camera and lens would have cost elsewhere. But all my other lenses are for film or full-frame digital cameras.
Only problem is, the lenses are probably all going to be redesigned by the time full-frame cameras are the norm.
The real limitation now is the quality of the lenses as the full frame sensors can resolve more than the lenses now.This wasn't an issue with film except for some real high resolution litho film in the past. But now, the sensors are so good, the lenses can't keep up.
Anyhow, for the prosumer, any of the good lenses now are more than enough and will be more than enough for the foreseeable future.