During the school year I carry 8gb worth of storage with me (1gb flash drive, 2gb flash drive, and 5gb micro drive).
I run all sorts of programs off my flash drive (Opera, Ad-Aware, Spybot, Winamp, VLC, OpenOffice, etc.). They are very useful when there are public PCs available for use.
Their flaw is speed. USB 2.0 is not fast enough. I recently bought a USB external HD and was very disappointed with how slow it is compared to my firewire 800 drives.
Until a much faster interface becomes popular, I don't think that you will see powerful programs like Inventor, Solidworks, 3dSMax, etc. be available for portable media.
Related but contradictory to my point, if you are interested in running any application from a portable device, check out
Mojopac.