I recently made a gold farm on a public smp server http://imgur.com/a/ziGZX#0
I covered surrounding areas in about 15,000 stone slabs to the point where there are on average 10-25 entities, in order for the gold trap to monopolize spawning in the area. However the trap only spawns a zombie pigman about once every 2 minutes. When I tested it in SP and MP on a private server the trap spawned pigmen once per second, so it's probably not a Minecraft issue, but a server one (although admins deny the existence of plugins that effect mob spawning).
However I cleared all possible spawns by nuking everything, instead of spamming half blocks when I privately tested it, so from here comes my question: Does the game attempt to spawn monsters on half block areas despite being unable to do so, and in so doing does not in essence favor the spawns offered by the trap? This is in contrast to just blowing up everything else so there are literally no spawn options at all for the game to consider except for the trap, which may explain why it's so successful when privately tested with everything nuked.
I simply can't bear placing another 15,000 half blocks to test this at the moment, even in creative. Thanks in advance for any answers.
Also I'm aware of the distance required of the player to allow mob spawning.
Mobs can spawn in a certain distance around every player. If you prevent mobs spawning near you then more mobs will spawn near other players. To make the farm work all other players must also be in mob free areas or offline.
You seem to have been pretty thorough and I can't think of anything else to suggest. The SMP server seems to have some odd issues in this regard. You're not the first one to observe this. That's one of the reasons why so many SMP players are excited about the merge in 1.3. because not only have a lot of things already been improved, but now the Mojang team will be focusing on just one client.