The T25 trimmer head does not fit the 128CD no matter what the Husqvarna catalog says. In any event I have replaced this with a Stihl and will not purchase any more Husqvarna products.
06-26-12 I just bought the bottom shaft from partstree.com You just can't buy the dust cap. You get part 16 the lower shaft, which includes part 21, part 22 the dust cap and a new trimmer head. It costs 20.95 plus shipping. I also bought the part 17 the kit shield to fit on my 128ld straight shaft. You maybe call first, but this works for my 2010 model.
Click on this web site: http://www.partstree.com/parts/?lc=husqvarna&mn=128+CD+%28USA%2C+Canada%29+%282010-03%29&dn=46540001
6-26-12 There are two different t25 cutting heads. The curved shaft has a Right hand thread and the straight has a t25 left hand thread, because of the gear box on the straight shaft 128ld, that turns counter clock wise, where the 128cd turns clockwise. They are two complete different arbor sizes, one LH and one RH.
Not sure what you are calling a "Dust Cap". The T-25 trimmer head comes in a couple of configurations. One is 10MM LH this is for straight shaft trimmers. The 3/8 RH is the trimmer head to fit the curved shaft trimmer you have. I would recommend purchasing the complete trimmer head.