I couldn't do without a laptop... I tried... but why try when it can just be both?
When I'm at home, laptop hooks into the monitor/keyboard and mouse and away I go. For meetings, I typically need my laptop so I unhook the peripherals and tada. I have all my working files right where I left them ready to show my client or work on at any time. My computer typically has Photoshop, Illustrator, Coda, Flash, Safari, Firefox, Mail and iTunes running at all times and sure sometimes I want more RAM... but that's my own fault... I'm only running 2 gigs. Maybe if I was into video editing, I would be singing a different tune, but for web development... a laptop is all I personally need.
It can function as a desktop... maybe not as fast as a Mac Pro... but oh well. I can handle the seconds and milliseconds difference that all these wonderful benchmarks show us.
I've been tempted... but reason always takes over.