Just tried 1.5.2 in my 1.5 world and noticed strange behavior of gravel blocks: gravel placed above air did not fall, also mining a block from below gravel block didn't cause it to fall.

Both things happened only once in a few trials. In both cases it was a client side problem only, as reloading the world fixed it.
Also I was exploring nether so I dont know if it happens in overworld too.

I have used 1.5.2 version downloaded through the new launcher, not through the links in the blog post (if that makes any difference).

Im going to test it further now.