For my personal development at home I use an old asus eeepc 900 as my tiny server. I've taken it apart to remove the screen and used ubuntu mini.iso to install CLI mode + apache2, MySQL and php5. I use apache with mod_vhost_alias to map a directory structure to hostnames automatically so developing/testing a project is very easy. My apache webroot folder is actually a folder in my dropbox, so my code is automatically backed up too.

I have a macbook (a 13" unibody) with the dropbox account and MAMP to develop on (type the code). I usually mount the folder of the project I'm working on, on the macbook and turn off dropbox to go easy on the syncing.

Lately I tend to go more to my linux roots (I was a linux only user for quite some years before I bought the macbook) so I bought an asus 1005P and put ubuntu on it, again with mod_vhost_alias and dropbox. I must admit it's a great little machine and it has almost 8hrs of battery time, wich is a real plus.

For me developing is something I should be able to do anywhere, at all time. Hence the rather unconventional development environment...