I couldn't find any other resource on this topic, so I wrote a simple guide as to what can pass through what. Everything here applies to extractors and assimilators as well.
First off, I found that when dealing with geysers, you can divide units into three size categories:
Note: Colossi are MEDIUM, archons are LARGE. I accidentally left them out of the picture.
Large units can never pass next to a vespene geyser or refinery if it is adjacent to another geyser, refinery, or cliffside. The notable unit here is siege tanks: although they appear to be smaller than immortals and queens, they actually are counted the same as thors and ultras.
Now, as for small and medium units. They behave mostly the same, except for one special case: diagonal cliffside + geyser, which I'll get to in a second. So, what exactly can small and medium units pass through?
Small/medium units CAN pass through two adjacent geysers, vertical or horizontal, OR a refinery and a geyser, stacked vertically (if the geyser is on the bottom):
Small/medium units can NOT pass through two adjacent refineries, vertical or horizontal, OR a refinery and a diagonal cliffside:
Small units CAN pass through a diagonal cliffside + a geyser, but medium units can NOT:
Small/medium units CAN pass through a vertical refinery + geyser, IF the geyser is on the bottom. If the geyser is on top, units can NOT pass through.
So now that we have this information, how can we use it? What sort of creative terrain can you make with adjacent refineries and geysers? Well, for one example, take an expansion like this:
It starts off as an island, blocked by two 500hp refineries with 1 armor. After destroying both refineries, you can fit workers, queens, and other small/medium units through. Now, you have a choice: you can take one or both geysers, turning the expansion back into an island, OR you can leave them unharvested, giving you a mineral-only expansion that can still be defended by marines/hydras/stalkers/etc. Of course, that's only one example.
Feel free to use this info without credit, I just felt like clearing up a somewhat nebulous issue in mapmaking. I think SC2's geyser behavior is much more interesting than BW's and could provide for some very unique maps.
Post corrections/feedback if needed!