Being a web developer, I must say the Dell Mini 9 may not be conventional (ie. being classed as a 'mere' netbook for web browsing & email) but it should definitely be looked at for it's sheer portability and when 'tweaked' for performance, etc, it can quite literally be a worthy supplement to you main computer (for me a MacBook alu model). Note, that when I do use the Mini 9, it is used both standalone AND with external monitor's & data projectors!
I particularly like its ability to run Dreamweaver & Fireworks CS4 (since we teach these apps), as well as text editors for straight coding. Dreamweaver CS4 has a nice feature where you can change the views at the touch of a button ie. go from design to coder, etc, this eliminating any concern about screen size.