It looks like Asus is making minor updates to a couple of entry level netbooks. The Eee PC 1001PQ, Eee PC 1001PX, and Eee PX 1005PX are all getting new names: Meet the Eee PC 1001PQD, 1001PXD, and 1005PXD.
As Netbook Italia points out, Asus reserves the letter D for models with Intel Atom N455 processors and support for DDR3 memory.
Aside from the DDR3 support, the Atom N455 chip is for all intents and purposes the same as the 1.66GHz Intel Atom N450 that was found in older versions of these netbooks, so you probably won't see much of a performance bump.
The change probably has more to do with Intel phasing out its older Atom chips and memory producers pumping out more DDR3 memory which will eventually lead to wider supply and lower prices for DDR3 memory than DDR2 RAM.
There's no word on when the new models will go on sale, but they're all listed on the Asus support site.
Asus bumps entry level netbook specs to include DDR3 memory is a post from: Liliputing