When you fall into the Void below the world, a very short period after you start falling you should take a large amount of damage, which under normal circumstances is enough to kill you. However, it only runs the damage once (that I know of, I don't willingly jump into the Void much), so if you manage to survive the hit, you will fall forever. You can survive it by wearing haxed armor (with ridiculously low damage values in the -32000s) or having a huge amount of health (over the 20 standard max).

Get NBTEdit from the Minecraft forums and open your save. There's a few ways to fix the problem of you falling. One, you could set your health variable to 0, which when you reload will instantly kill you and trigger a respawn. Two, you could set the DeathTime variable to 22 (or so), which regardless of health tells Minecraft that you are dead and need to respawn. Or three, you could look up your player position variables and your spawn point variables, and set the player position to the spawn point.

But more to the point, if you're on Alpha singleplayer, you really should have died upon falling into the Void. These fixes could be a bit glitchy, but they *should* get you back to the surface plus or minus your inventory.