Can anybody remember the name of this story? I remember the plot vaguely: a couple's dead son returns, and they are very happy, and than it turns out that it was an alien transforming into different human shapes according to the memories of people... I am not even sure it's Ray Bradbury - but it feels like him... Please help!

Well, there are chapters in 'The Martian Chronicles' that sound familiar to your description...

Masha,
'Tis, in fact, the story �The Martian� from Martian Chronicles.

Pazhalsta
(tried typing Russian characters, but, like the character in this story, their shape changed!)

[This message has been edited by Braling II (edited 08-01-2005).]

i also read a story called Mars is Heaven but couldn't remember who that was by. it is similar in that these people go on a spaceship to mars but when they get there they see all their passed on loved ones and realize that mars is heaven.

Those are both among Ray's Martian stories.