The Web 2.0 | Office 2.0 word-processor that does the fundamentals right, and also makes the online-offline experience very seamless, will take most of the pie...
"If online services that do not work offline go down — for maintenance or whatever reason — the outage should be considered as a detriment, not a growing pain."
MJ Foley | All About Microsoft (Aug 23, 2007)
Mary Jo has it sideways: it's not about being down, per se; it's about moving in & out of range of the access point -- a much more frequent occurrence. The ubiquity of wifi is a myth and seamless intercourse between environments and data when connected and dis-connected will make and break the Office 2.0 aspirant.
If offline scenarios are not a core part of your dev roadmap and not factored in your buz model, go home now!