Warning: This is pure speculation. I have no inside information and no affiliation with Google or its employees.
I don't think it will be long before Gmail's website is available offline for PC and Mac: http://gears.google.com/.
Why would they put so much energy into developing client-side data storage libraries if they weren't planning to use them? And certainly searching the entire Internet offline isn't going to work (unless you have a REALLY BIG hard drive) - so that only leaves a few remaining services that are useful offline:
- Gmail (obviously)
- Contacts (obviously)
- Calendar (obviously)
- Docs & Spreadsheets (probably)
- Blogger (possibly)
Google has a lot of other services in addition to those - but I see the above as being most likely, and in the order presented.
I haven't gotten around to writing my thoughts on Gmail in general, but for now please accept this post as a blanket "thumbs-up" for the Gmail experience. Honestly, I am all kinds of giddy about the near-term possibility of in-browser, offline Gmail.
My fingers are crossed and my eyes peeled for additional developments in this arena.