FWIW I've been running two Pi's 24/7 - one since just before Xmas 2012 (you can prolly guess why), and one from about a month ago. All(?) my transient files are held in ram - either in a 32MB ramdisk I've made, or another chunk of ram that's used for all the /var/log stuff. I'm not using fstrim, primarily because I'd not thought of it...
Both Pi's are headless, and both use 4GB SD cards.
The first one crashes (or more precisely reboots on a kernel panic or when the wifi drops and cannot reconnect) a few times a day. It also runs a webserver ...
.... so the restarts could be caused by traffic(?). Its card is a Transcend 4GB Class 6 SDHC.
The second Pi is not accessible to the outside world (it's on the dark web), and its uptime is faultless.
That one uses a Hama 4GB Class 10 SDHC card.
Both cards have been tested with fsck, as in I've put the cards in a different PC, made sure they're not mounted, and executed these commands on both cards:
sudo fsck /dev/mmcblk0p1 -v
sudo fsck /dev/mmcblk0p2 -v
BTW, I'm assuming that the Class is the number inside the Big C on the physical card label, but I could well be wrong on this one.