On Diamond Pipe Distribution: I admit, the last time I tried using a Diamond Pipe in this manner was way back in SSP Technic for Minecraft 1.2.5. I'm not even sure which version of BuildCraft I was using; perhaps they've improved the functionality since then. My setup (and problem) was this: I had an assembly line of Automatic Crafting Tables to make something that needed an equal number of Sticks and Planks. ACT #1 made Planks from Logs, meaning I had to send exactly 1 out of every 3 Planks to a Stick-making ACT to get an equal number of each for my final recipe. But a Diamond Pipe with two Planks in one output and one Plank in another resulted in that fuzzy "almost right" distribution that kept mucking up my ACTs. (Ultimately, I had to use that Distribution Pipe from the mod addon I mentioned to get it to work.)
On RedPower Pneumatic Tube Distribution: I know that round-robin distribution by RP Pneumatic Tubes is not limited to immediately adjacent inventories; I recently tested this before setting up a system that relied on it. My test had a Transposer pulling Cobblestone, one-at-a-time, from a full Chest (2304 Cobblestone). The Cobblestone would hit an intersection of Pneumatic Tubes that led in opposite directions to two Barrels separated by 9 Pneumatic Tubes, with the intersection in the middle: (B) - - - - + - - - - (B). At the intersection, the Cobble alternated direction one after the other in perfect round-robin fashion. When the Chest was empty, each Barrel had exactly 1152 Cobblestone. This was a pretty simple test, though, so while it proved that inventories don't need to be immediately adjacent for distribution to work, it doesn't rule out other sources of interference (a larger network, or the presence of Restriction Tubes, perhaps?)