I think aircards are great but personally I would prefer tethering because:
1) less hardware to carry, I already have my MB and iPhone with me most of the time.
2) WiFi is pretty much available whereever I go, but for those places that it is not availble and I would like to have access, I have the option to tether. That is a huge bonus. I
I wont care to much about speed, I just would like to have the capabilty. For me tethering would totally help. Most of my work is on my iDisk anyhow so when I really need the web it is to get something or sync my iDisk.