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Step One - Applying modifiers to the enchantment level
The first thing that Minecraft does is apply two modifiers to the base enchantment level. Each modifier is restricted to a certain range, with numbers close to the middle of the range more common than those near the ends.
The first modifier is based on the item's "enchantability," which depends on the material and the type of the item (see the table below). Minecraft picks a number between 0 and the enchantability, then adds that number plus one to the enchantment level. Bows have an enchantability of 1 for this purpose.
Shoot...SCORE!
Source
For fun, I figured I'd test this out. It would be understandable to nerf enchanting a little, I suppose, because of all the ways available to gain exp now. (breeding, fishing added with 1.4.2)
Fortunately, I can't seem to find anything to suggest it's been nerfed. A good googlin' gives me nothing to suggest the nerf either. Just now I received, with L26 enchants on all:
An iron axe got efficiency 4, unbreaking 3, fortune 2.
Iron sword: smite 3, looting 2.
Iron pickaxe: unbreaking 3. Eh.
Iron shovel: fortune 2, unbreaking 3.
Last night I received sharpness 3+ fire 1 on my diamond sword with a level 26 enchant as well.
Mojang's good with telling us these things, I think it's safe to say there's been no nerf.
Best Answer
Enchantments are completely random. There is no way to guarantee which enchantment you will get.
From the wiki
If you don't want to waste enchantments on equipment, I would suggest trying to enchant books instead. This is slightly more resource intensive, but that way, later you can combine books in an anvil to get stronger enchantments, and even apply a specific set of enchantments of your choosing to the equipment you want to enchant.
Post-Note: I have actually found that if you are looking for a bow with the "Infinity" Enchantment specifically, you will actually have better luck with fishing.