"Yep, offline capability is a key checklist for the next generation of Web apps, particularly for things like Web mail (when you're on the plane and have to get work done.)"
Hmm. Doesn't that assumes that inflight Web access will never be available or too expensive?
Anyway, when I'm on the road and I won't have Wifi access to the Internet I just download my personal email to my PDA, and my work email is already to sync'd on my laptop. Its already pretty easy.
Now this is not to say that I'm not convinced that offline won't be a key capability on the checklist as Hinchcliffe suggests, but I just see us cycling back into resource hungry, difficult to install and maintain, fat-clients that we went online to avoid in the first place... I just hope in all of this we remember to KISS.