I have a "seriously tore up" 15" MBP from spring 2008 which I carry with me for really mobile use, and a 17" 2010 MBP which sits in my "base" location for weeks at a time. I went that route, vs. the 15" HD, because I can't easily have an external monitor while being mobile -- otherwise, the 15" HD seems to be the best, coupled with an external 24, 27, or 30.

I am considering getting a 13" MBP or maybe an x301 or panasonic s9 whenever they next get bumped. I don't care for netbooks (too weak), but a long battery life small laptop would be a nice tool. The lack of a keyboard on the ipad cripples me; often, the blackberry is more useful.