I tested this with NeoKnight on a Black/White wifi double battle, and did some testing myself on Platinum/HeartGold double battle.
First, the basic way Pressure works is that any move targeted at the Pokemon with Pressure uses an extra PP - i.e. 2 PP instead of 1 PP. So using Tackle against Weavile would use 2 PP, but using Tackle against Weavile's teammate (without Pressure) would only use 1 PP.
In a double battle, moves targeted at the Pressure Pokemon use 2 PP and moves targeted at the other Pokemon use 1 PP as normal.
If both opposing Pokemon have Pressure, multi-target moves lose an extra PP for every one they hit. So Neo's Archeops used Rock Slide against my Weavile and Dusknoir and it lost 3 PP!
Attacking an ally with Pressure only uses 1 PP in Generation 5, but 2 PP in Generation 4. In B/W, my Mamoswine using Earthquake against the opponents and hitting my teammate Weavile used 1 PP each time. In Pt/HG attacking an ally and both opposing pokemon with Pressure loses 4 PP (1 PP for the move plus 1 for each Pressure Pokemon, including the ally).
Status moves are also affected if they target a Pressure Pokemon, e.g. Hypnosis on Giratina. Using something on yourself (e.g. Agility) is not affected.
Interestingly, it doesn't affect moves like Reflect even though it supposedly targets the user and its ally (who had Pressure).
