I have an inspiron 5150 and to be honest I am not that impressed...
I got it 2.5 years ago(almost top of the line then)...its a P4 2.8GHz CPU, 512 Ram, 64MB vid, and its not great...id stay away from dells...that thinkpad may be ugly but it works and its NICE and its only 5.6lbs...
and from what you say you want powerful mobility...
I know, here is one i built quick on Dell.com, I dont know if this link works but the Inspiron 1505 isnt too bad if you go to the one with the free 2GB ram upgrade and better CPU upgrade