Yes, same here, especially when travelling, and when wifi or 3G is not reliably available (typically driving round the west of Ireland). Having my current list of key contact emails and to do emails available offline on the iPad has been a lifesaver. I archive everything worth keeping at home on the PC in folders. I do keep sync with the webmail server to delete (archive) when I delete from the iPad while away though. Then the PC only downloads relevant items when I get back. I can imagine other kinds of individual flexibility.
I still have the same question I had the beginning, is there any way at all (not one suggestion yet) to back up the mail content of the Mail application on the iPad? If not, any idea why not? If yes, how?