Personally, I need a solution that works on Linux, Windows, and Android. iPhone/iPad would be nice too, but I don't have those (although my wife does, so compatability there is a plus).

Truecrypt has Linux and Windows covered. It has not been ported to Android yet. A few other products are attempting to offer Truecrypt support on Android, but from my research they have limitations that are not appealing to the general public (like you phone has to be rooted, no GUI - commandline only, etc.)

I have found one product that works fantastically on Linux/Windows/Android/iPhone, and that is "KeePass". But it is an app designed to manage passwords, not general purpose documents. Still, if password management is what you need, this app can't be beat. It is known as "KeePass" on Windows, "KeePassX" on Linux and MacOS, "KeePassDroid" on Android, and "iKeePass" on iPhone/iPad. http://keepass.info/download.html