In my experience with generating new worlds and finding the stronghold is close, they've almost always been in a swamp biome. Is this intentional, a bug, or just a coincidence?
Well, according to the wiki article, it's totally random:
This wasn't enough to definitively conclude anything, though, so I decided to look into the source code. Here's a snippet:
That code determines the allowed biomes for a stronghold. You'll notice that it includes a great variety of biomes. (This code is from 1.1, so jungle isn't in there, but it probably is in 1.2.3.)
The rest of the code is fairly uneventful, and includes no biasing or weighting to a certain biome. So no, swamps have the same probability as any other biome to generate a stronghold.
Upon experimentation, I have concluded that there is no one biome that spawns strongholds more. I loaded 3 randomly generated maps, which generated strongholds in the Extreme Hills biome, River biome, and Snow biome. The fact that before, I kept finsing them in the swamp was a coincidence.
Also, from the wiki:
Suggesting that because they generate at random, there is no way to predict the biome in which they generate. Essentially, they generate in the same way that villages, dungeons, and abandoned mine shafts generate.