MLC uses two cells to represent each bit, but can pack the cells more densely. MLC requires 2 writes for each bit.

SLC uses one cell per bit so it writes faster and has no "partially programmed" state. 

Found this facinating tidbit in the SLAX forum: http://www.slax.org/forum.php?action=view&parentID=1633&highlight=usb%20lifetime

Here is even more detail:  http://www.supertalent.com/datasheets/SLC_vs_MLC%20whitepaper.pdf

Why do we care?

I'm hoping we don't.  One of the technical challenges we have to overcome is making the Key robust even if its removed abruptly without proper shutdown.   We might want to try SLC USB Keys and see if it makes any difference.  The cost doesn't look that bad. Google shopping has a 1GB stick for about $10.

This is mostly an interesting tidbit and a note for later. At this point I don't know that using SLC will be required.