12'' isn't really can get much power and AutoCAD on 12'', painful for eyes.
12.1'' and less for <3k is around hp dv2, compaq cq20, acer 2930, culv notebook, and netbook.

better consider dell studio 15, good after sales, good performance for the price, or benq s42, better spec, still under 3k.

culv notebook is almost like netbook size, but using intel SU-series processor, which is basically underclocked and undervolted core 2 duo.
which performance wise, its between atom and regular core 2 duo.
example, which under 12'' is acer 1410T, acer 1810T, hp dm1, dell 11z, asus ul20a. These are around 1.5kg and give around 5++ hours battery life, for AutoCAD, better get the one with su4100 or su7300, these are dual core, much better performance for multitasking.
else, stick with regular notebook, which should be around 2kg, acer 2930, compaq cq20.
all those still under 3k.

ah, forgotten acer ferrari one, yeah this one should have better overall performance, battery life around 2-4 hours.
hp dv2 as well too, but its run quite hot and battery life is not that good.

the one I suggested all is using intel gma 4500mhd, which less performance but less heat.

acer ferrari one get my pick if not needed 6++ hours of battery life.

sufficiently not so sure, slower yeah. also depends how complex the project is.
but you got not much choice for 12'' and below.
most of them is intel gma 4500mhd, except dv2 comes with ati 3410, ferrari one comes with hd3200.

well, they are more meant for browsing, office, and such. not really for powerful job.
but again, your choice will really be limited.
best gpu on 12'' would be dv2, but the processor still lacking power.
cpu wise, su4100, su7300 might be better, but these usually paired with intel gma4500mhd.

I suggest u stick with C2D 2.0Ghz + 13 inch laptop. Your main concern is the weight not screen size right? Now 13" weight just 1.6kg and more powerful. My friend just sold his CULV 1.2Ghz laptop and he is AutoCAD user too. This processor keep show "No Responding" in AutoCAD and slow.

inspiron 13 with p8700 and hd4330 for rm 2999, have much better performance ^^