Wear leveling and using ERaser on Flash
Yes it is a problem. The flash controller thats in the card will determine exacly what sectors of physical flash are written to. For example, say eraser is used with 7 passes to "overwrite" a particular sector of flash memory. The controler can erase the desired sector only once and then spread the remaining 6 overwrites to different physical sectors to spread out the "wear" on the flash drive.
I spoke with a SanDisk rep today and he indicated that they have started to drop support for wear leveling. You are correct about the taking 85 years to reach 1,000,000 erase/write cycles... but only if you write to a sector once a day. There are many functions that write the same file, to the same location hundreds or even thousands of times a day.
SanDisk refers to the flash for these application as hi-rel (High Reliability).
So any flash memory that has wear leveling cannot be gauranteed to be overwritten as many times as you think unless the entire flash is overwritten. Again, if you are truly paranoid about this subject, read " Data Remanence in Semiconductor Devices" by Peter Gutmann. It is a very good summary of how data can be recovered from RAM or Flash, but also illustrates the difficulty that it would take to do so. It has a simplified intro to Semiconductor Physics so you can understand the difference between RAM and Flash memory and why each acts the way it does.
For the super paranoids, use Eraser to write random data 50-100 passes to the ENTIRE Flash disk BEFORE you store any files on it. Read the paper to figure out why. Then, if you MUST destroy the files that were stored on it, Overwrite the entire flash drive with several passes of random data. Any wear leveling could spread the passes out over the entire disk... maybe.
If you are ultranoid, use a belt sander to grind the flash drive to powder. Collect this dust and combine it with the shreddings from your NSA certified paper shredder. Burn the entire collection until it is carbon, then mix the ashes with water and pour down the drain. Anyone that recovers data after that, deserves it.