Without wanting to go into detail (so as not to spoil anything specific):

Is Odin's enchantment on Mjolnir (Whosoever holds this hammer...) still in effect?

yepper i do believe so

It hasn't come up in ages, in Thor comics, at least. I don't know if anyone's tried to pick it up in an Avengers book lately, though, or somewhere like that.

At least up until the aftermath of Fear Itself, it was almost-certainly in effect. Donald Blake would still strike his cane on the ground and exchange places with Thor and Mjolnir, and that was always related to the original enchantment. Now, it probably is, but might not be.

It might be addressed in some capacity when the new Thor, God of Thunder series starts, written by Jason Aaron.

I really loved when Thor started up. He didn't do his usual thing, he kicked Stark's ### all over Nawlins and then he got #####ed by a new writer and fear itself. He goes from being the KING of Asgard to once again being daddy's peon, then to being a dead peon....Then come's back to life? How did THAT work? Did they bring him back just for AvX?

I'd like to see him back as the Silver Age Superman of Marvel. The guy you tested nuclear bombs against because you knew it wouldn't put a dent in him.

The Avengers Dissassembled and then relaunch was pretty good at putting him back at being the supreme butt kicker in Marvel. The guy who can fend off Galactus solo and about whom even the other Norse gods say 'as powerful as Odin' or that 'even Odin fears to challenge him' as Stan Lee intended.