There's still a few things I need my laptop (an 11" MBA) for, such as running Parallels, and advanced Keynote presentations with looping video and other features not yet available in the iPad version of Keynote.
My desktop can of course do that, too, but I can't lug a desktop to give a lecture off of. Many of my lectures I give via iPad and the HDMI adapter, but some still require my laptop.
But... my laptop usage is probably only 25% of what it used to be, maybe less, since I got the iPad. I go weeks without touching the laptop these days. iPad gets used daily.