Didn't help me. Not in SP, at least.
I started seeing this in 1.3.1, possibly due to a major change that "Made Singleplayer internally use a Multiplayer server".
The "ssp" mod link I posted above re enables single-player mode and resolves this issue, but I don't think it is compatible with 1.5 yet. You can always roll Minecraft back to 1.4.7 and use the mod there.
@thefoot
This has been an issue for quite a while. You can use this mod to fix Single Player:
http://www.minecraftforum.net/topic/534818-152-olddaysnbxlite-spawnhuman-ssp-sspc/
(Very first post, "Daily builds" section, "SSP") It's working 100% perfectly for me in 1.6.2 - I haven't upgraded yet to avoid the hassle of re-installing the mod, but the guy who makes it is usually pretty good with keeping it up-to-date.
When you install it correctly, you'll see a "Play SMP" button on the "Select World" page. DON'T press it - start the game normally.
I'm not sure what effect this mod has on Multi-Player.