I think you might find this interesting:
Intel Atom vs. SU2300 vs. SU3500 ULV Processors | Netbook Computers
What you trade off with the dual core is power consumption. I've had HP laptops and they tend to be optimistic with battery estimates. So, if they say "up to 5 hours" you can pretty much count on 3 in real world use. I can attest to the longer battery life of the Acer ... and mine is the dual core SU9200.
Some of the other considerations would be that the DM1 has an 11" screen where the Aspire has a 14" screen, but (and this would be the killer for me) the Aspire comes with Vista, where the HP has Windows 7. Vista wasn't really as bad as the reviews made it out to be, but 7 is that much better. Plus the HP has a year warranty where the Aspire (reboxed) does not.