It can't. This is exactly because of the wear-leveling mechanism: the controller tries to balance all writes so that all physical blocks have around the same amount of writes happened to them and this is done by mapping a physical block to a virtual one. When the OS tries to write block #15, it might actually be writing to physical block #3.

It would be possible, but would require some tinkering around in Maemo OS. Basically, the directory /home/user needs to be encrypted and this could be solved by creating a, let's say, 1 gigabyte file on MMC. Then set up a loopback device pointing to that file, make an encrypted filesystem on it, and mount it on /home/user. The problem is obviously that this would have to happen before 'user' logs in, ie. so early in the boot process that no application yet runs as 'user'.