There are a few line of sight algorithms in the articles section I believe...

Firstly, you were right about pixel stuff being way too slow, and that triangle thing was a good thought but completely out of the question.
You''d better hope you are doing a tile-based game, or you only want your los to apply to objects and not the enviorment. Basicly, it is very simple... every time you want to blit something (perhaps a tile or an enemy or whatever) you first check to see if the pleyer can ''see'' it. If he can, then go ahead and blit. How do we know if the player can see something? Like this...
Two ways, I sugest you use a combination of both for what you want.
For that triangle thing, for each object or tile you blit you check the difference between the dirrection the player is facing and the direction from the player to the thing we are blitting. If the angle difference is bellow, say, 45 degrees, then we go ahead and blit. This is how I would do the triangle thing.
The other way is to check if a straight line between the player and the object/tile is blocked. If it is, then the player cannot ''see'' the thing so we do not blit it. However, I suggest checking if the lines are blocked on a tilemap based level for the sake of speed, and if your not using a tilemap I''m sure you''ll think of something.
For what you want, I suggest using a combination of both methods - first check the angle difference, the go on to check the straight line thing.
Of course, your line of sight will be totally ''blocky'' ie you see something one moment then the next it is completely gone, but that is about the best you can do in DDraw.
Note that this is how I do it, and there is quite possibly a better way, considering that this method comes straight from my own creation.
If you need help, post back. I have all the code you would need. And for God''s sake, checking the difference between two angles does NOT mean simply subtracting one from the other.

Well it certainly doesnt look great... but it works well speed-wise. I use the strait-line-blocked thing in my tile based game and it works perfectly with no drop in the framerate whatsoever. I blit about 300 tiles to my screen at once and, as I said, no speed cost - suiprising when I have a 450 mhz PC with less than 64 megs of ram.
To disguise the blockyness of los on tiles, you could hold the los thing in an array, run a check, and blit edges for the los onto the back buffer.

Geee... my game that I am working on (with my los) is getting fairly large... uses DDraw. I suppose I could give you some screenshots demonstrating los. E-Mail adress? I use hotmail, and maximum sending thing is 1 meg, see why I cant send the whole thing?

Just sent the screenshots. The only thing I ask is that you dont take any of my graphics... not that you would, but I should cover myself. Let me know if you need any more help.