With both my previous computer (running on a ATI X1800XT) and my current upgraded computer (running on a ATI HD 4850), the in-game anti-aliasing checkback is unselectable. I suppose I could enable it in my drivers, but I prefer to do it in-game wherever possible. Does anyone else have this option selectable or unselectable?

I've not been able to find that option. Can you point me to where it's hiding, then I can take a look.
UPDATE:
Found it. What settings are you using when you start Sacrifice? Direct3D T&L HAL?

Yep. It wouldn't be worth it to sacrifice (pun intended) transform & lighting capabilities just for in-game antialiasing, if that's what enables the option.

Try it anyway.. that way we have an answer for anyone else too.

I just tried without T&L, and the box was still untickable.
EDIT: Randomly omitting words makes things hard to read.
EDIT 2: Both I and the OP have ATI cards. Anyone else want to pitch in with their card type and whether or not the box is tickable?

Geforce 8600M GT- Tickable
Geforce 8600 GT- Tickable
Geforce 9800 GT (SLI) - Tickable

T&L or not seems to have nothing to do with it. It's still untickable for me. I even tried changing around the various graphics settings, including the resolution, and it was still unselectable.
I suppose it's only fair that nVIdia cards would work better with some older games, since quite a few work better on ATI cards. And this is a relatively minor annoyance, since the game still looks good without AA (especially for being 8 years old). Forcing AA in the drivers may work almost as well anyway.

on my ATI cards, if I enable anti-aliasing in the control panel (ati tray tools in my case) then I can, indeed, select anti aliasing.
If it's application controlled in ati tools, then I can't select it.
cheers
DrWig

I'm not saying you're wrong, but that's completely illogical. Selecting Anti-Aliasing in the control panel forces AA on, while choosing Application Controlled lets the game choose whether to use it (and at what level). They're supposed to be completely separate from each other... if you have AA enabled in a game, you can still force it on in the drivers.
I'll investigate this further on my system. I don't have ATI Tray Tools, so it will be interesting to see how it works with the Catalyst Control Center.

I tried it with the Catalyst Control Center, and you were right. That's the only time I've seen a game behave the opposite of how it should. The weirdest part of it is that with AA forced on outside of the game, you would think AA would be selected (and not just selectable) automatically. I suppose the age of this game and its early AA support has something to do with this odd behavior. I'm not sure when games started supporting AA, but Sacrifice is probably one of the earliest ones (it's just a simple toggle, after all, instead of a switch between AA levels).
I didn't load a map to see if it made much difference either. Maybe at some later date I'll investigate further and take screenshots to compare. Right now X3: Terran Conflict is keeping me busy.

I loaded and took screenies etc.
TLDR version: It looks worse with AA forced.
-The big jaggies (such as pyro's shrine in the ethereal realm) are still present, but the others are gone (i.e. AA is doing its job).
So, the side-effects;
-The (main) menu interface looked better, even though AA shouldn't be doing anything there. The background also turned black when I selected single player, for some reason (but not multi or load game...). In-game menus suffer from the next point as far as text is concerned.
-Text goes weird and blocky ... it's difficult to describe, but kind've what text might look like if your resolution was set to about 320x240.
-When there would usually be a fade-out (i.e. closing a game, load screen start/end, etc), the screen instead turns black.
-Clashes between different sections of the environment become apparent - zipper-ish lines between different sections of sky. This is especially noticeable in the ethereal realm..
-The # of colours seems to be decreased?
-Your UI starts looking weird/pixelated.
For the record, I'm using a mobility HD 2600.
At a guess, I'd say shiny just hasn't optimised the game at all for ATI cards.
I'd post screenies, but don't know of a free program that can convert .tga to .jpg.

Use IrfanView - it's the last image program you'll ever need!

Cheers.
Okay, screenies. The environmental effects are probably the most obvious in elysium, and the least obvious in the glebe.
No AA: http://img510.imageshack.us/img510/2309/noaakd2.jpg
2x AA: http://img522.imageshack.us/img522/383/2xaalm9.jpg
4x AA: http://img185.imageshack.us/img185/8997/4xaaaf6.jpg
8x AA: http://img266.imageshack.us/img266/2435/8xaafp8.jpg
To compare, the sky with and without AA
No AA: http://img356.imageshack.us/img356/1232/skynoaasi7.jpg
8x AA: img=http://img65.imageshack.us/img65/3241/skyaaem6.jpg
EDIT: Screenies are 1024x768, machine's graphics card is a Mobility Radeon HD 2600, Catalyst driver, updated (7.14.0010.0523 if anyone cares), Aniso filtering is off, v-sync is on.

You're right - it does look worse. I guess it's better to play with jaggies.
Of course, it's possible that it looks better with AA on with Nvidia cards.