The OLPC computer
with SSD and cheap components is designed to last at least 5 years.
Here is an interesting article
about using flash memory for swap space. Flash memory is supposed to level out writes over available space. One observation in the article is that, assuming a life of 100,000 writes, and completely overwriting available space every hour, it should still last eleven years. Add in some sophisticated error-correction, and it could be much longer. It would still be a good idea to tune the o/s to avoid unnecessary system-related writes.
I would think the biggest problem will be getting hold of an eee at a price that makes it look like a worthwhile alternative to a standard notebook PC.