I read on the Windups site that they have a guaranteed amount of times u can write to them before they start to deteriorate (like CDRW's) not sure how many though, its got to be in the 1,000's I would have thought. Thats why Wind' has a function to save desktop rather than doing it each time u move an icon, change background etc.
That would mean they would die out with extensive play in like 6 months because games and emulators read and write to the smc many times during gameplay.
In real use ´heavy´ use of the SMC will make it last for about 5 to 7 years. Which is more than enough because at that time we will all be using our PSP, GP64 or whatever that will almost surely does not use SMC.
Also, after 100,000 cyces, only the sectors that have actually gone through those cycles die off (well not exactly then....) that mens that instead of completely shutting down, the card has less and less space on it after time.