The only thing I would need on the iPad to be a complete desktop replacement:
Adobe Photoshop, or a powerhouse photo creator / editor app that I can do my graphics design projects on.
Everything else I would do using it: Email, Internet browsing, Music, Youtube, and I have my iPhone for calling.
I dont have an iPad, but I do have an iPhone and when my desktop graphics card was acting up, I actually used my iphone to control it. All I had to do was remember the button presses in order to get the desktop up and connected to the internet (I have Linux installed so I have to scroll down the GNU Grub menu, enter my password and press a few buttons to connect to the internet manually)
So for my internet surfing and emailing needs, I just used the apps and for other things like saving, reformatting and rendering my projects I just did it using LogMeIn Ignotion (a VNC app).
At one point I even logged on my PC through my iphone FROM my friends house, opened up iTunes and did a sync for all my music wirelessly (Cydia extension, PwnTunes) and began playing the music through his speakers!