I've got an 8gb SD card I'm not using, and want to try something new with. Namely, use it as the root partition on a laptop with a built-in card reader.

I only found one (outdated) entry on the Arch Wiki, and other searches haven't turned up much of use for achieving this, except that it has to be set up at the system install.

What I want to know is what deviations from a standard install of Arch this would entail, what the risks are (with /home mounted separately) in doing so, and what potential benefits would be achieved in using the card for the root partition.

Also, if a standard (IE, HD partition based) install were to exist as well, would it be safe/wise/possible to have Pacman use a single partition for packages for both HD and SD installs, or is keeping the two separate a better idea?

I did this with several distros on my Asus EEE PC (not Arch specifically) back when I was deciding which distro to choose. If your BIOS supports boot from an SD card, then it is just another drive, and no special steps or precautions are necessary. Performance is, of course, very slow.

Now my Dell Mini on the other hand simply doesn't support boot from SD. Really it depends on your BIOS...

Personally I think it makes more sense to install the operating system to an internal drive and use the card reader for additional storage. Putting the root partition on SD means the computer is unbootable if you remove the card...

Since neither the card nor reader see much use anyway, it's unlikely to be removed in any case.
I don't know exactly if my laptop can boot direct from it or not; I haven't looked yet.

I thought that GRUB (not 2) from a HD-based /boot partition might be able to load it, but I've yet to find anything in GRUB's documentation to confirm this. Still looking though.

I also thought that booting from an SD card actually gained performance, not lost it - perhaps I misinterpreted the benefits...

So your plan is to have /boot and /home on the internal HDD and / (root) on the SD card? That should work I suppose, not something I would personally be interested in, but Linux is infinitely flexible.

That's the theory, at least. Just as soon as I get Wine to cooperate with the last two apps I need Windows for, I can remove it's partition and reclaim a good 10GB of space from that and tidy up the HD a bit more.
The final layout after that would be:
/ - on the SD card
/home, /boot - on the HD

Possibly /usr on a separate partition, but like sticking / on the SD, that mostly depends on theif the ups are better than the downs.

Edit: After a little investigating, it turns out /var would be beneficial on the HD rather than SD because it gets used so much.

10gb should be plenty of space to put / on the internal HDD too (unless you plan to install a ton of applications).

I know, that's what I plan to turn into a kind of secondary install - a kind of just-in-case for if something happens to the card itself.