Second on the Lenovo X series. I prefer the T series, which to me is mobile enough - but it's a rather large laptop in comparison.
One of my on-site datacenter guys carries an X-series around, and these guys are *hard* on their laptops. Dell/etc. crud might last a year if you're lucky - but the Lenovo's last 3-4 years regularly.
As someone long ago once told me - Once you go Thinkpad you never go back
The only other laptop I would consider for someone who actually uses it like a laptop (e.g. travels with the thing regularly) would be a Toughbook.
-Phil