There is a "spawn point", but you actually spawn anywhere within about 30 blocks of it. Try deopping and trying to damage the environment, wherever you can't touch is part of spawn. The "spawn point" will be in dead center.
You can use this to build a "spawn area".
Update: According to Notch's blog post about changes coming in 1.6, the torches-on-fast-leaves bug is getting fixed:
- Fixed buttons and levers being placeable on leaves in “fast graphics” mode
This means that monsters can no longer spawn on leaves, either, as they are always considered transparent, even when using Fast graphics.
Monsters can spawn on any solid, non-transparent block which is at light level 7 or lower.
Signs, torches, rails, and the like are ignored when spawning monsters; they'll spawn on the block underneath just fine. This is why one-block-wide railways need to be lit.
Transparency means whether or not light passes through a block rather than whether you can see through it. Half-blocks, doors, stairs, and glass all count as transparent and prevent monster spawning. Leaves, oddly, count as transparent only when Fancy graphics are enabled. An easy way to test a block for transparency is to try placing a torch or sign on it — neither can be placed on transparent blocks. (Meaning that torches can be placed on leaves with Fast graphics, but not with Fancy.)
Torches put out light level 14, so monsters can't spawn within 5 blocks of a torch orthogonally or 3 blocks diagonally. If you put your torches in a diamond pattern, they can be up to 9 blocks apart, but it's best to move them closer than that on uneven terrain. (I use 5 myself.)
If you like, you can actually torch your treetops by changing your graphics to Fast, placing the torches, and switching back. Your torches will stay rooted, even though you wouldn't normally be able to place them that way. Personally, I think it makes them look rather cheerful. :-)
Best Answer
I suspect this has been asked before, but I can't find the question so I'll answer this one:
Aggressive monsters, with the exception of ghasts, will spawn when the light level is at 7 or below. Torches are considered a light source of level 14, Lava, Glowstone, and Fire are at level 15. For each square away from the light source, you drop the light level by 1. If multiple light sources are affecting that square, the highest light level is taken. Glass does not lower light levels as it passes through (anymore than the normal amount), water and ice adds an additional 2 levels of penalty.
Monsters also cannot spawn within 24 squares of the player, and monsters will not spawn if you go to sleep in a bed to skip the night. If every player in SMP is in bed by the time night comes around, then the night will be skipped.
Finally, keep in mind that spiders can now climb walls so even though you have built a wall around your house, you may still find spiders from within. So make sure that you build a roof.