yes, I also wonder what the longevity of the card will be, being that Im booting from the mmc card. But the way I see it, it should last just as long as the internal memory, since it is solid state as well! And the card is much easier to replace than the internal memory! (Until 2gb cards aren't available anymore)
Fast or slow shouldn't make a difference, as its the number of read/write cycles that counts. High speed will stress the card more though, so lf there is a manufacturing defect, it could break sooner in high speed.