I use remember the milk on my pc and iphone and it has an offline option which I use.
(i have a desktop icon for RTM too, I use linux tho so may be different for you)
Plus I have it sync up with Evolution - which is my email, which also syncs to google calendar, which also syncs to my iphone calendar.
Overall i have things that sync up pretty well, plus I use the emal reminders features and I have to say even when I moved and then my office fell through leaving me with vurtually no internet for 3 weeks - I could use stuff.
I also use dropbox, which again has an iphone app - between the things I have it worked for me at the moment.
As for security, if its encrypted on transmission, and then encrypted at server so even the server owner cannot tell (without breaking the encryption) whats there - then that is secure enough for me, and apparently for my data protection obligations too.
Dropbox is great, because all my data files are in the folder, not only does it back up to the cloud but the data is on my 2 other machines as well