I've been waiting for people to start getting and reviewing their Dell machines with Ubuntu before ordering mine. There's a nice article on the Ubuntu Forums:Review of Dell XPS 410n with Ubuntu Feisty preinstalled. Also good coverage of the Dell E1505n laptop.
I'm not ready to convert my 4-year-old Inspiron laptop over to Ubuntu because of suspend/hibernate and wireless issues but it sounds better with more modern machines. Maybe if Dell or HP or Lenovo comes out with a backlit LED screen like Apple's I'll be ready to switch. Also liked the tips on wireless with Ubuntu here.