Years ago i read about this with cd/dvd. It was something like a 100 yrs for a rewritable.
Dont worry about it. flash drive will work fine. never lost one and then again you will not loose a hd and or micro drive in the same day. ya have plenty of time to back up on a new hd if lost. On the other hand, a microdrive is portable and you can carry with you.
Just get one big enough.
Most CDs/DVDs can loose they're data within a few years. There are special ones though that are meant for long term storage, such as the Memorex Gold line which are rated for 100 years. Flash drives are better but still have a small chance of failure. If you want to back something up and not have to worry about it, use an online storage service such as DropBox.