I've tried also with php 5.1.1 for debian and didn't help either :)

Assigned to the maintainer.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

There is no bug here, the return value of imap_fetch_overview() is a complex array that contains objects within it. If you trully want to "free" it you need to itterate through the array freeing each element individually.

Well.... I've tested in another way by putting imap_fetch_overview in a function. When the function ends the memory should be free right?

Not in this case.

<?php
	echo "Memory usage: " . memory_get_usage() . "\t- Before fetch_overview function\n";			
	testclear ($stream);
	echo "Memory usage: " . memory_get_usage() . "\t- After fetch_overview function\n\n";			

function testclear($stream) {
	$imapOverview = imap_fetch_overview($stream, "1:*");
}
?>

My result is still the same:
Memory usage: 50264	- Before fetch_overview function
Memory usage: 89936	- After fetch_overview function

Memory usage: 89800	- Before fetch_overview function
Memory usage: 94608	- After fetch_overview function

Memory usage: 94472	- Before fetch_overview function
Memory usage: 97232	- After fetch_overview function

Memory usage: 97096	- Before fetch_overview function
Memory usage: 97232	- After fetch_overview function

I had also unset each object from the result array and it did not help anything

Putting the code into the function would not force the freeing of memory in this case. Unless you explicitly free it yourself you'll need to wait till the script termination time.
By defention a leak in PHP would be a situation where at the end of the script's execution memory allocated for the purpose of running the script was not freed.