I run a Raspberry Pi as a 24/7 Bitcoin node.
I don't know that Bitcoin itself can wear down an SD card (I'm offering no proof here, just my speculation), but the high memory requirements of bicoind might create swapping that
will wear it down.
By default, Raspian has a swap file enabled. I disabled it by simply removing the execute privileges of the startup script, /etc/init.d/dphys-swapfile.
sudo chmod -x /etc/init.d/dphys-swapfile
I solved the problem of swap by buying a sata to usb cable (
http://www.amazon.com/C2G-Cables-30504-Serial-Adapter/dp/B000UO6C5S/ref=sr_1_2?ie=UTF8&qid=1383363496&sr=8-2&keywords=sata+adapter) and simply hooking up an old 160GB internal laptop HDD. I formatted the HDD as ext4 with a swap partition, added both to /etc/fstab, and got it up and running smoothly.
The cord I linked to also provides external power, so don't worry about the Pi not being able to supply enough.
edit: It might also help to use the least amount of video memory possible if you plan on running the node without GUI. You can do this by running
sudo raspi-config
and choosing the memory split option to change the video memory to the lowest possible.