Is anyone else having troubles installing the unofficial patch by drog?
The installer tells me that it has trouble writing to Arcanum.exe and a number of other files. I've tried re-installing the entire game to no avail even though all the other mods (high-resolution, etc) seem to be working fine.

Guessing you are running Vista/7 and have it installed under C:\Program Files\GoG.com\Arcanum?
If so then it may be a UAC issue, have you tried running the UAP in Admin mode?
Personally I would suggest installing the game outside of program files (like in c:\GoG.com instead - I install GoG games to my D drive).

Admin mode worked perfectly - thank you so much for the quick reply :)

No problem :)
Keep that tip in mind any time you install a patch/mod/etc that modifies core files (that are protected by UAC) in the Program Files folder - with UAC on you need to ensure you run those patch/mod installers as admin or it will fail.

Can you point me toward a verified place to download the high res mod? In all the places I've found it, it's just missing.

I found a pretty good site eventually

hxxp://www.terra-arcanum.com/downloads/section/arcanum
replace the "x"s with "t"s obviously, unsure what the rules are on live links here

that has the UAP, hi res mod, and a few others