Fans to the rescue. Thanks to a modder named Drog, we not only have a fully working widescreen patch, but a huge fan patch that fixes tons of issues. And the great thing is, it adds nothing that wasn't already in the game, just re-enables some things that seem to have been accidentally left out, like extra dialogue options, voice clips, and items.
WARNING: While I and many others have played through the game with these patches, they are technically unofficial, and the modder in question has said he is finished with Arcanum modding, so expect no support. Also, you MUST apply them before you start a new game, to avoid issues with incompatible saves.
Apply these in order from top to bottom:
The latest version (12/28/2009) of the fan patch. Essential. Must have. Get it here.
High-quality townmaps (Better looking in-game maps, also fixes some potential missing map errors):
And the High Resolution Patch. Weidu-based, so follow the directions to set it up for your system. Will allow you to run Arcanum at pretty much ANY resolution, provides an optional "compact" UI and optional high-res fonts:
And that should set you up real nice. If you want more info on Arcanum, or any of the other Troika games, head to http://www.terra-arcanum.com and have a look around. Note that they also have some other mods and things for Arcanum, but most of them significantly alter the core game, or are incompatible with the UAP bugfixes. You might want to give the high-quality music patch a spin, but the original music sounds excellent and I wager you won't be able to tell much of a difference. For the purest and most hassle-free experience I recommend you stick with these patches for your first playthrough.
EDIT: If you have any problems with not being able to download, try refreshing the download page, or try the alternate links at this forum post (thanks sanpats), or for the 91225 update and High-res patches try these alternate links, courtesy of Petrell: UAP91225 update and High-res patch.
EDIT 2: Removed old patch version, as apparently version 91225 is stand-alone. Vagabond says it is anyway. Blame him if it breaks.
EDIT 3: Removing contributed torrent link due to torrent site jumping the shark.